diff --git a/src/ifcviewer-full/Federation.cpp b/src/ifcviewer-full/Federation.cpp
new file mode 100644
index 0000000000..a72f783103
--- /dev/null
+++ b/src/ifcviewer-full/Federation.cpp
@@ -0,0 +1,308 @@
+/********************************************************************************
+ *                                                                              *
+ * This file is part of IfcOpenShell.                                           *
+ *                                                                              *
+ * IfcOpenShell is free software: you can redistribute it and/or modify         *
+ * it under the terms of the Lesser GNU General Public License as published by  *
+ * the Free Software Foundation, either version 3.0 of the License, or          *
+ * (at your option) any later version.                                          *
+ *                                                                              *
+ * IfcOpenShell is distributed in the hope that it will be useful,              *
+ * but WITHOUT ANY WARRANTY; without even the implied warranty of               *
+ * MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the                 *
+ * Lesser GNU General Public License for more details.                          *
+ *                                                                              *
+ * You should have received a copy of the Lesser GNU General Public License     *
+ * along with this program. If not, see <http://www.gnu.org/licenses/>.         *
+ *                                                                              *
+ ********************************************************************************/
+
+#include "Federation.h"
+
+#include <QDir>
+#include <QFile>
+#include <QFileInfo>
+#include <QSaveFile>
+#include <QJsonArray>
+#include <QJsonDocument>
+#include <QJsonObject>
+#include <QJsonValue>
+#include <QUuid>
+
+namespace {
+constexpr const char* kSchema = "ifcfed/1";
+
+QString resolvePath(const QString& fed_dir, const QString& stored) {
+    if (stored.isEmpty()) return stored;
+    QFileInfo fi(stored);
+    if (fi.isAbsolute()) return QDir::cleanPath(stored);
+    return QDir::cleanPath(QDir(fed_dir).absoluteFilePath(stored));
+}
+
+// Returns abs_path relative to fed_dir if abs_path lives under fed_dir,
+// otherwise returns abs_path unchanged.
+QString relativizePath(const QString& fed_dir, const QString& abs_path) {
+    QString fed_canon = QDir::cleanPath(fed_dir);
+    QString abs_canon = QDir::cleanPath(abs_path);
+    if (!fed_canon.endsWith('/')) fed_canon += '/';
+    if (abs_canon.startsWith(fed_canon)) {
+        return QDir(fed_canon).relativeFilePath(abs_canon);
+    }
+    return abs_canon;
+}
+}  // namespace
+
+Federation::Federation(QObject* parent) : QObject(parent) {}
+
+QString Federation::generateId() {
+    return QUuid::createUuid().toString(QUuid::WithoutBraces);
+}
+
+bool Federation::isFederationPath(const QString& path) {
+    return path.endsWith(".ifcfed", Qt::CaseInsensitive);
+}
+
+void Federation::clear() {
+    file_path_.clear();
+    name_.clear();
+    created_ = QDateTime();
+    modified_ = QDateTime();
+    models_.clear();
+    has_home_view_ = false;
+    home_view_ = HomeView{};
+    setDirty(false);
+}
+
+void Federation::markClean() {
+    setDirty(false);
+}
+
+void Federation::setDirty(bool d) {
+    if (dirty_ == d) return;
+    dirty_ = d;
+    emit dirtyChanged(d);
+}
+
+const Federation::Model* Federation::findById(const QString& fed_id) const {
+    for (const auto& m : models_) {
+        if (m.id == fed_id) return &m;
+    }
+    return nullptr;
+}
+
+QString Federation::addModel(const QString& source_path,
+                              const QString& display_name) {
+    if (source_path.isEmpty()) return {};
+    if (isFederationPath(source_path)) return {};  // no nested federations
+
+    Model m;
+    m.id = generateId();
+    m.display_name = display_name.isEmpty()
+        ? QFileInfo(source_path).fileName()
+        : display_name;
+    m.source_kind = "local";
+    m.source_path = QDir::cleanPath(QFileInfo(source_path).absoluteFilePath());
+    models_.push_back(std::move(m));
+    setDirty(true);
+    return models_.back().id;
+}
+
+void Federation::removeModel(const QString& fed_id) {
+    for (auto it = models_.begin(); it != models_.end(); ++it) {
+        if (it->id == fed_id) {
+            models_.erase(it);
+            setDirty(true);
+            return;
+        }
+    }
+}
+
+void Federation::setHomeView(const HomeView& hv) {
+    home_view_ = hv;
+    has_home_view_ = true;
+    setDirty(true);
+}
+
+void Federation::clearHomeView() {
+    if (!has_home_view_) return;
+    has_home_view_ = false;
+    home_view_ = HomeView{};
+    setDirty(true);
+}
+
+bool Federation::load(const QString& path,
+                      QStringList* warnings,
+                      QString* err) {
+    QFile f(path);
+    if (!f.open(QIODevice::ReadOnly)) {
+        if (err) *err = QString("Cannot open %1: %2").arg(path, f.errorString());
+        return false;
+    }
+    QByteArray bytes = f.readAll();
+    f.close();
+
+    QJsonParseError pe;
+    QJsonDocument doc = QJsonDocument::fromJson(bytes, &pe);
+    if (doc.isNull() || !doc.isObject()) {
+        if (err) *err = QString("Parse error in %1: %2").arg(path, pe.errorString());
+        return false;
+    }
+    QJsonObject root = doc.object();
+
+    clear();
+    file_path_ = QDir::cleanPath(QFileInfo(path).absoluteFilePath());
+    QString fed_dir = QFileInfo(file_path_).absolutePath();
+
+    QString schema = root.value("schema").toString();
+    if (schema != kSchema && warnings) {
+        *warnings << QString("Unknown schema '%1' (expected '%2'); attempting to load anyway.")
+                         .arg(schema, kSchema);
+    }
+
+    name_ = root.value("name").toString();
+    created_ = QDateTime::fromString(root.value("created").toString(), Qt::ISODate);
+    modified_ = QDateTime::fromString(root.value("modified").toString(), Qt::ISODate);
+
+    QJsonArray arr = root.value("models").toArray();
+    for (int i = 0; i < arr.size(); ++i) {
+        if (!arr[i].isObject()) {
+            if (warnings) *warnings << QString("models[%1] is not an object; skipping.").arg(i);
+            continue;
+        }
+        QJsonObject mo = arr[i].toObject();
+
+        Model m;
+        m.id = mo.value("id").toString();
+        if (m.id.isEmpty()) m.id = generateId();
+        m.display_name = mo.value("display_name").toString();
+
+        QJsonObject so = mo.value("source").toObject();
+        m.source_kind = so.value("kind").toString("local");
+        if (m.source_kind != "local") {
+            if (warnings)
+                *warnings << QString("models[%1]: unsupported source kind '%2'; entry kept but not loaded.")
+                                 .arg(i).arg(m.source_kind);
+            // Keep raw stored path so save() round-trips correctly.
+            m.source_path = so.value("path").toString();
+            models_.push_back(std::move(m));
+            continue;
+        }
+
+        QString stored = so.value("path").toString();
+        if (stored.isEmpty()) {
+            if (warnings) *warnings << QString("models[%1]: missing source.path; skipping.").arg(i);
+            continue;
+        }
+        m.source_path = resolvePath(fed_dir, stored);
+
+        if (m.display_name.isEmpty())
+            m.display_name = QFileInfo(m.source_path).fileName();
+
+        QJsonValue tv = mo.value("transform");
+        if (tv.isArray()) {
+            QJsonArray ta = tv.toArray();
+            if (ta.size() == 16) {
+                for (int k = 0; k < 16; ++k) m.transform[k] = float(ta[k].toDouble());
+                m.has_transform = true;
+            } else if (warnings) {
+                *warnings << QString("models[%1]: transform must be 16 floats; ignored.").arg(i);
+            }
+        }
+
+        QJsonValue vv = mo.value("visible");
+        if (vv.isBool()) m.visible = vv.toBool();
+
+        models_.push_back(std::move(m));
+    }
+
+    QJsonValue hv = root.value("home_view");
+    if (hv.isObject()) {
+        QJsonObject ho = hv.toObject();
+        QJsonArray ta = ho.value("target").toArray();
+        HomeView v;
+        if (ta.size() == 3) {
+            v.target = QVector3D(float(ta[0].toDouble()),
+                                 float(ta[1].toDouble()),
+                                 float(ta[2].toDouble()));
+        }
+        v.distance = float(ho.value("distance").toDouble(50.0));
+        v.yaw      = float(ho.value("yaw").toDouble(45.0));
+        v.pitch    = float(ho.value("pitch").toDouble(30.0));
+        home_view_ = v;
+        has_home_view_ = true;
+    }
+
+    setDirty(false);
+    return true;
+}
+
+bool Federation::save(const QString& path, QString* err) {
+    QString abs_path = QDir::cleanPath(QFileInfo(path).absoluteFilePath());
+    QString fed_dir = QFileInfo(abs_path).absolutePath();
+
+    QJsonObject root;
+    root["schema"] = kSchema;
+    if (!name_.isEmpty()) root["name"] = name_;
+
+    if (!created_.isValid()) created_ = QDateTime::currentDateTimeUtc();
+    modified_ = QDateTime::currentDateTimeUtc();
+    root["created"]  = created_.toUTC().toString(Qt::ISODate);
+    root["modified"] = modified_.toUTC().toString(Qt::ISODate);
+
+    QJsonArray arr;
+    for (const auto& m : models_) {
+        QJsonObject mo;
+        mo["id"] = m.id;
+        mo["display_name"] = m.display_name;
+
+        QJsonObject so;
+        so["kind"] = m.source_kind;
+        if (m.source_kind == "local") {
+            so["path"] = relativizePath(fed_dir, m.source_path);
+        } else {
+            // Round-trip raw value for unsupported kinds.
+            so["path"] = m.source_path;
+        }
+        mo["source"] = so;
+
+        if (m.has_transform) {
+            QJsonArray ta;
+            for (float v : m.transform) ta.append(double(v));
+            mo["transform"] = ta;
+        }
+        if (!m.visible) mo["visible"] = false;
+
+        arr.append(mo);
+    }
+    root["models"] = arr;
+
+    if (has_home_view_) {
+        QJsonObject ho;
+        QJsonArray ta;
+        ta.append(double(home_view_.target.x()));
+        ta.append(double(home_view_.target.y()));
+        ta.append(double(home_view_.target.z()));
+        ho["target"]   = ta;
+        ho["distance"] = double(home_view_.distance);
+        ho["yaw"]      = double(home_view_.yaw);
+        ho["pitch"]    = double(home_view_.pitch);
+        root["home_view"] = ho;
+    } else {
+        root["home_view"] = QJsonValue();  // null
+    }
+
+    QSaveFile f(abs_path);
+    if (!f.open(QIODevice::WriteOnly | QIODevice::Truncate)) {
+        if (err) *err = QString("Cannot write %1: %2").arg(abs_path, f.errorString());
+        return false;
+    }
+    f.write(QJsonDocument(root).toJson(QJsonDocument::Indented));
+    if (!f.commit()) {
+        if (err) *err = QString("Failed to commit %1: %2").arg(abs_path, f.errorString());
+        return false;
+    }
+
+    file_path_ = abs_path;
+    setDirty(false);
+    return true;
+}
diff --git a/src/ifcviewer-full/Federation.h b/src/ifcviewer-full/Federation.h
new file mode 100644
index 0000000000..21bb6ba3bd
--- /dev/null
+++ b/src/ifcviewer-full/Federation.h
@@ -0,0 +1,105 @@
+/********************************************************************************
+ *                                                                              *
+ * This file is part of IfcOpenShell.                                           *
+ *                                                                              *
+ * IfcOpenShell is free software: you can redistribute it and/or modify         *
+ * it under the terms of the Lesser GNU General Public License as published by  *
+ * the Free Software Foundation, either version 3.0 of the License, or          *
+ * (at your option) any later version.                                          *
+ *                                                                              *
+ * IfcOpenShell is distributed in the hope that it will be useful,              *
+ * but WITHOUT ANY WARRANTY; without even the implied warranty of               *
+ * MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the                 *
+ * Lesser GNU General Public License for more details.                          *
+ *                                                                              *
+ * You should have received a copy of the Lesser GNU General Public License     *
+ * along with this program. If not, see <http://www.gnu.org/licenses/>.         *
+ *                                                                              *
+ ********************************************************************************/
+
+#ifndef FEDERATION_H
+#define FEDERATION_H
+
+#include <QObject>
+#include <QString>
+#include <QStringList>
+#include <QDateTime>
+#include <QVector3D>
+
+#include <array>
+#include <vector>
+
+// In-memory representation of an .ifcfed file (IFC federation).
+//
+// A federation is a named, ordered list of model sources plus an optional
+// "home view" camera state.  Source paths can be relative (resolved against
+// the .ifcfed's directory) or absolute.  Save() reserialises paths relative
+// when they live under the federation file's directory tree, absolute
+// otherwise — Save As recomputes against the new location.
+//
+// Round-trip-only fields today (no UI to edit, but preserved across load/
+// save): per-model `transform` (4x4, column-major), per-model `visible`,
+// future cloud `source.kind`s.
+class Federation : public QObject {
+    Q_OBJECT
+public:
+    struct HomeView {
+        QVector3D target;
+        float distance = 50.0f;
+        float yaw = 45.0f;     // degrees
+        float pitch = 30.0f;   // degrees
+    };
+
+    struct Model {
+        QString id;                       // stable, persisted
+        QString display_name;
+        QString source_kind = "local";    // future: "http", "speckle", ...
+        QString source_path;              // resolved absolute when kind == "local"
+        bool has_transform = false;
+        std::array<float, 16> transform{}; // column-major; identity when !has_transform
+        bool visible = true;
+    };
+
+    explicit Federation(QObject* parent = nullptr);
+
+    // Round-trip
+    bool load(const QString& path, QStringList* warnings, QString* err);
+    bool save(const QString& path, QString* err);
+
+    // Mutations
+    void clear();
+    QString addModel(const QString& source_path,
+                     const QString& display_name = QString());
+    void removeModel(const QString& fed_id);
+    void setHomeView(const HomeView& hv);
+    void clearHomeView();
+
+    // Accessors
+    const std::vector<Model>& models() const { return models_; }
+    const Model* findById(const QString& fed_id) const;
+    bool isDirty() const { return dirty_; }
+    void markClean();
+    QString filePath() const { return file_path_; }
+    QString name() const { return name_; }
+    bool hasHomeView() const { return has_home_view_; }
+    const HomeView& homeView() const { return home_view_; }
+
+signals:
+    void dirtyChanged(bool dirty);
+
+private:
+    void setDirty(bool d);
+    static QString generateId();
+    static bool isFederationPath(const QString& path);
+
+    QString file_path_;
+    QString name_;
+    QDateTime created_;
+    QDateTime modified_;
+    std::vector<Model> models_;
+    bool has_home_view_ = false;
+    HomeView home_view_;
+    bool dirty_ = false;
+};
+
+#endif // FEDERATION_H
diff --git a/src/ifcviewer-full/MainWindow.cpp b/src/ifcviewer-full/MainWindow.cpp
index e36af95670..3105af7cf5 100644
--- a/src/ifcviewer-full/MainWindow.cpp
+++ b/src/ifcviewer-full/MainWindow.cpp
@@ -19,11 +19,13 @@
 
 #include "MainWindow.h"
 #include "AppSettings.h"
+#include "Federation.h"
 #include "SettingsWindow.h"
 #include "LodBuilder.h"
 #include "SidecarCache.h"
 
 #include <QApplication>
+#include <QCloseEvent>
 #include <QMenuBar>
 #include <QFileDialog>
 #include <QFileInfo>
@@ -40,6 +42,11 @@ MainWindow::MainWindow(QWidget* parent)
     setupUi();
     setupMenus();
 
+    federation_ = new Federation(this);
+    connect(federation_, &Federation::dirtyChanged, this, [this](bool dirty) {
+        setWindowModified(dirty);
+    });
+
     loader_ = new SceneLoader(viewport_, this);
     connect(loader_, &SceneLoader::loadStarted,
             this, &MainWindow::onLoadStarted);
@@ -82,7 +89,7 @@ MainWindow::MainWindow(QWidget* parent)
         if (!show) stats_label_->clear();
     });
 
-    setWindowTitle("IfcViewer");
+    updateWindowTitle();
     resize(1400, 900);
 }
 
@@ -132,12 +139,32 @@ void MainWindow::setupUi() {
 
 void MainWindow::setupMenus() {
     auto* file_menu = menuBar()->addMenu("&File");
-    auto* open_action = file_menu->addAction("&Add Files...", this, &MainWindow::onFileOpen);
-    open_action->setShortcut(QKeySequence::Open);
+    file_menu->addAction("&New Federation",
+                         this, &MainWindow::onFederationNew,
+                         QKeySequence::New);
+    file_menu->addAction("&Open Federation...",
+                         this, &MainWindow::onFederationOpen,
+                         QKeySequence::Open);
+    file_menu->addSeparator();
+    file_menu->addAction("&Add Files...",
+                         this, &MainWindow::onFileOpen,
+                         QKeySequence("Ctrl+Shift+O"));
     file_menu->addAction("Add &Database...", this, &MainWindow::onDatabaseOpen);
+    file_menu->addSeparator();
+    file_menu->addAction("&Save Federation",
+                         this, &MainWindow::onFederationSave,
+                         QKeySequence::Save);
+    file_menu->addAction("Save Federation &As...",
+                         this, &MainWindow::onFederationSaveAs,
+                         QKeySequence::SaveAs);
+    file_menu->addSeparator();
     file_menu->addAction("&Settings...", this, &MainWindow::onFileSettings);
     file_menu->addSeparator();
     file_menu->addAction("&Quit", QKeySequence::Quit, qApp, &QApplication::quit);
+
+    auto* view_menu = menuBar()->addMenu("&View");
+    view_menu->addAction("Set &Home View", this, &MainWindow::onSetHomeView);
+    view_menu->addAction("&Go to Home View", this, &MainWindow::onGoHomeView);
 }
 
 void MainWindow::onFileOpen() {
@@ -170,18 +197,189 @@ void MainWindow::onFileSettings() {
 }
 
 void MainWindow::addFiles(const QStringList& paths) {
+    QStringList accepted_paths;
+    QStringList accepted_fed_ids;
+    for (const auto& p : paths) {
+        QString fed_id = federation_->addModel(p);
+        if (fed_id.isEmpty()) continue;  // .ifcfed or empty path — silently skipped
+        accepted_paths << p;
+        accepted_fed_ids << fed_id;
+    }
+    loadModelsFromPaths(accepted_paths, accepted_fed_ids);
+    updateWindowTitle();
+}
+
+void MainWindow::loadModelsFromPaths(const QStringList& paths,
+                                      const QStringList& fed_ids) {
+    if (paths.isEmpty()) return;
     auto ids = loader_->addFiles(paths);
     for (int i = 0; i < paths.size() && i < static_cast<int>(ids.size()); ++i) {
-        uint32_t id = ids[i];
+        uint32_t mid = ids[i];
+        const QString& fed_id = fed_ids[i];
+        fed_id_to_model_id_[fed_id] = mid;
+        model_id_to_fed_id_[mid] = fed_id;
+
         QString display = QFileInfo(paths[i]).fileName();
         auto* root = new QTreeWidgetItem(element_tree_);
         root->setText(0, display);
         root->setText(1, "IFC Model");
         root->setData(0, Qt::UserRole, static_cast<uint32_t>(0));
-        tree_roots_[id] = root;
+        tree_roots_[mid] = root;
     }
 }
 
+bool MainWindow::openFederation(const QString& path) {
+    if (!confirmDiscardIfDirty()) return false;
+
+    QStringList warnings;
+    QString err;
+    if (!federation_->load(path, &warnings, &err)) {
+        QMessageBox::warning(this, "Open Federation",
+                              QString("Could not open federation:\n%1").arg(err));
+        return false;
+    }
+
+    clearScene();
+
+    QStringList paths;
+    QStringList fed_ids;
+    QStringList missing;
+    for (const auto& m : federation_->models()) {
+        if (m.source_kind != "local") continue;  // already warned by load()
+        if (!QFileInfo::exists(m.source_path)) {
+            missing << m.source_path;
+            continue;
+        }
+        paths << m.source_path;
+        fed_ids << m.id;
+    }
+    loadModelsFromPaths(paths, fed_ids);
+
+    for (const auto& msg : missing) {
+        warnings << QString("Source not found, kept in federation: %1").arg(msg);
+    }
+    if (!warnings.isEmpty()) {
+        QMessageBox::warning(this, "Open Federation",
+                              "Federation opened with warnings:\n\n" + warnings.join("\n"));
+    }
+
+    federation_->markClean();
+    updateWindowTitle();
+
+    if (federation_->hasHomeView()) {
+        const auto& hv = federation_->homeView();
+        viewport_->setCamera(hv.target.x(), hv.target.y(), hv.target.z(),
+                             hv.distance, hv.yaw, hv.pitch);
+    }
+    return true;
+}
+
+void MainWindow::onFederationNew() {
+    if (!confirmDiscardIfDirty()) return;
+    clearScene();
+    federation_->clear();
+    updateWindowTitle();
+}
+
+void MainWindow::onFederationOpen() {
+    QString path = QFileDialog::getOpenFileName(
+        this, "Open Federation", QString(),
+        "IFC Federation (*.ifcfed);;All Files (*)");
+    if (path.isEmpty()) return;
+    openFederation(path);
+}
+
+bool MainWindow::onFederationSave() {
+    if (federation_->filePath().isEmpty()) return onFederationSaveAs();
+    QString err;
+    if (!federation_->save(federation_->filePath(), &err)) {
+        QMessageBox::warning(this, "Save Federation",
+                              QString("Could not save federation:\n%1").arg(err));
+        return false;
+    }
+    updateWindowTitle();
+    return true;
+}
+
+bool MainWindow::onFederationSaveAs() {
+    QString suggested = federation_->filePath();
+    if (suggested.isEmpty()) suggested = "federation.ifcfed";
+    QString path = QFileDialog::getSaveFileName(
+        this, "Save Federation As", suggested,
+        "IFC Federation (*.ifcfed);;All Files (*)");
+    if (path.isEmpty()) return false;
+    if (!path.endsWith(".ifcfed", Qt::CaseInsensitive)) path += ".ifcfed";
+
+    QString err;
+    if (!federation_->save(path, &err)) {
+        QMessageBox::warning(this, "Save Federation",
+                              QString("Could not save federation:\n%1").arg(err));
+        return false;
+    }
+    updateWindowTitle();
+    return true;
+}
+
+void MainWindow::onSetHomeView() {
+    auto cs = viewport_->cameraState();
+    Federation::HomeView hv;
+    hv.target   = cs.target;
+    hv.distance = cs.distance;
+    hv.yaw      = cs.yaw;
+    hv.pitch    = cs.pitch;
+    federation_->setHomeView(hv);
+    updateWindowTitle();
+}
+
+void MainWindow::onGoHomeView() {
+    if (!federation_->hasHomeView()) {
+        status_label_->setText("No home view set for this federation.");
+        return;
+    }
+    const auto& hv = federation_->homeView();
+    viewport_->setCamera(hv.target.x(), hv.target.y(), hv.target.z(),
+                         hv.distance, hv.yaw, hv.pitch);
+}
+
+void MainWindow::clearScene() {
+    while (!tree_roots_.empty()) {
+        uint32_t mid = tree_roots_.begin()->first;
+        viewport_->removeModel(mid);
+        removeModelUi(mid);
+    }
+    fed_id_to_model_id_.clear();
+    model_id_to_fed_id_.clear();
+}
+
+bool MainWindow::confirmDiscardIfDirty() {
+    if (!federation_->isDirty()) return true;
+    auto ret = QMessageBox::question(
+        this, "Unsaved Federation",
+        "The current federation has unsaved changes. Save before continuing?",
+        QMessageBox::Save | QMessageBox::Discard | QMessageBox::Cancel,
+        QMessageBox::Save);
+    if (ret == QMessageBox::Cancel) return false;
+    if (ret == QMessageBox::Save) return onFederationSave();
+    return true;  // Discard
+}
+
+void MainWindow::updateWindowTitle() {
+    QString fed_path = federation_->filePath();
+    if (fed_path.isEmpty() && federation_->models().empty()) {
+        setWindowTitle("IfcViewer");
+    } else if (fed_path.isEmpty()) {
+        setWindowTitle("untitled[*] — IfcViewer");
+    } else {
+        setWindowTitle(QFileInfo(fed_path).fileName() + "[*] — IfcViewer");
+    }
+    setWindowModified(federation_->isDirty());
+}
+
+void MainWindow::closeEvent(QCloseEvent* event) {
+    if (confirmDiscardIfDirty()) event->accept();
+    else                          event->ignore();
+}
+
 void MainWindow::onLoadStarted(uint32_t /*mid*/, QString display_name) {
     progress_bar_->setValue(0);
     progress_bar_->setVisible(true);
@@ -326,6 +524,12 @@ void MainWindow::writeSidecarForModel(uint32_t mid) {
 }
 
 void MainWindow::removeModelUi(uint32_t mid) {
+    auto fed_it = model_id_to_fed_id_.find(mid);
+    if (fed_it != model_id_to_fed_id_.end()) {
+        fed_id_to_model_id_.erase(fed_it->second);
+        model_id_to_fed_id_.erase(fed_it);
+    }
+
     auto root_it = tree_roots_.find(mid);
     if (root_it != tree_roots_.end()) {
         delete root_it->second;
diff --git a/src/ifcviewer-full/MainWindow.h b/src/ifcviewer-full/MainWindow.h
index b616293d59..13dfecf061 100644
--- a/src/ifcviewer-full/MainWindow.h
+++ b/src/ifcviewer-full/MainWindow.h
@@ -34,6 +34,7 @@
 #include "ViewportWindow.h"
 #include "SceneLoader.h"
 
+class Federation;
 class SettingsWindow;
 
 class MainWindow : public QMainWindow {
@@ -43,12 +44,22 @@ public:
     ~MainWindow();
 
     void addFiles(const QStringList& paths);
+    bool openFederation(const QString& path);
     void setPendingCamera(const QString& params);
     void setPendingBenchmark(int frames);
 
+protected:
+    void closeEvent(QCloseEvent* event) override;
+
 private slots:
     void onFileOpen();
     void onDatabaseOpen();
+    void onFederationNew();
+    void onFederationOpen();
+    bool onFederationSave();
+    bool onFederationSaveAs();
+    void onSetHomeView();
+    void onGoHomeView();
     void onFileSettings();
     void onObjectPicked(uint32_t object_id);
     void onTreeSelectionChanged();
@@ -69,6 +80,11 @@ private slots:
 private:
     void setupUi();
     void setupMenus();
+    void loadModelsFromPaths(const QStringList& paths,
+                             const QStringList& fed_ids);
+    void clearScene();
+    bool confirmDiscardIfDirty();
+    void updateWindowTitle();
     void populateProperties(uint32_t object_id);
     void appendElementToTree(uint32_t model_id,
                              uint32_t object_id,
@@ -84,6 +100,7 @@ private:
 
     ViewportWindow* viewport_ = nullptr;
     SceneLoader*    loader_   = nullptr;
+    Federation*     federation_ = nullptr;
     SettingsWindow* settings_ = nullptr;
     QWidget* viewport_container_ = nullptr;
     QTreeWidget* element_tree_ = nullptr;
@@ -95,6 +112,11 @@ private:
     // Per-model tree roots, keyed by model_id.
     std::map<uint32_t, QTreeWidgetItem*> tree_roots_;
 
+    // Bidirectional federation_id <-> model_id map.  Federation owns the
+    // persistent ids; SceneLoader owns the runtime model_ids.
+    std::unordered_map<QString, uint32_t> fed_id_to_model_id_;
+    std::unordered_map<uint32_t, QString> model_id_to_fed_id_;
+
     // Display-side element registry for tree + property lookup.
     std::unordered_map<uint32_t, ElementInfo> element_map_;
     std::unordered_map<uint32_t, QTreeWidgetItem*> tree_items_;
diff --git a/src/ifcviewer-full/main.cpp b/src/ifcviewer-full/main.cpp
index d15a525c53..8f861e3f31 100644
--- a/src/ifcviewer-full/main.cpp
+++ b/src/ifcviewer-full/main.cpp
@@ -40,7 +40,9 @@ int main(int argc, char* argv[]) {
     QCommandLineParser parser;
     parser.setApplicationDescription("IfcOpenShell IFC Viewer");
     parser.addHelpOption();
-    parser.addPositionalArgument("files", "IFC file(s) to open", "[files...]");
+    parser.addPositionalArgument("files",
+        "IFC file(s) and/or one .ifcfed federation to open",
+        "[files...]");
     parser.addOption({{"c", "camera"},
         "Set camera: tx,ty,tz,dist,yaw,pitch", "params"});
     parser.addOption({{"b", "benchmark"},
@@ -51,9 +53,17 @@ int main(int argc, char* argv[]) {
     window.show();
 
     auto args = parser.positionalArguments();
-    if (!args.isEmpty()) {
-        window.addFiles(args);
+    QStringList file_args;
+    QString fed_arg;
+    for (const auto& a : args) {
+        if (fed_arg.isEmpty() && a.endsWith(".ifcfed", Qt::CaseInsensitive)) {
+            fed_arg = a;
+        } else {
+            file_args << a;
+        }
     }
+    if (!fed_arg.isEmpty()) window.openFederation(fed_arg);
+    if (!file_args.isEmpty()) window.addFiles(file_args);
 
     if (parser.isSet("camera")) {
         window.setPendingCamera(parser.value("camera"));
diff --git a/src/ifcviewer/ViewportWindow.cpp b/src/ifcviewer/ViewportWindow.cpp
index 81d75aebd7..e928525f03 100644
--- a/src/ifcviewer/ViewportWindow.cpp
+++ b/src/ifcviewer/ViewportWindow.cpp
@@ -1186,6 +1186,10 @@ QString ViewportWindow::cameraString() const {
         .arg(camera_pitch_, 0, 'f', 2);
 }
 
+ViewportWindow::CameraState ViewportWindow::cameraState() const {
+    return { camera_target_, camera_distance_, camera_yaw_, camera_pitch_ };
+}
+
 void ViewportWindow::keyPressEvent(QKeyEvent* event) {
     const int key = event->key();
 
diff --git a/src/ifcviewer/ViewportWindow.h b/src/ifcviewer/ViewportWindow.h
index 68e9893020..ec223c559f 100644
--- a/src/ifcviewer/ViewportWindow.h
+++ b/src/ifcviewer/ViewportWindow.h
@@ -172,6 +172,14 @@ public:
     void setBenchmarkFrames(int n);
     QString cameraString() const;
 
+    struct CameraState {
+        QVector3D target;
+        float distance;
+        float yaw;    // degrees
+        float pitch;  // degrees
+    };
+    CameraState cameraState() const;
+
     struct FrameStats {
         float fps;
         float frame_time_ms;