diff --git a/src/solvers.jl b/src/solvers.jl
index da3491a..677de75 100644
--- a/src/solvers.jl
+++ b/src/solvers.jl
@@ -1,10 +1,6 @@
 # This file is a part of JuliaFEM.
 # License is MIT: see https://github.com/JuliaFEM/JuliaFEM.jl/blob/master/LICENSE.md
 
-# Solver stuff
-
-abstract Solver
-
 """
 Solve field equations for a single problem with some dofs fixed. This can be used
 to test nonlinear element formulations. Dirichlet boundary is assumed to be homogeneous
@@ -55,7 +51,7 @@ end
 
 
 """ Simple linear solver for educational purposes. """
-type LinearSolver <: Solver
+type LinearSolver
     name :: ASCIIString
     field_problems :: Vector{Problem}
     boundary_problems :: Vector{BoundaryProblem}
@@ -138,3 +134,221 @@ function call(solver::LinearSolver, time::Float64)
     return norm(u)
 end
 
+
+# Tuple{Symbol,Any,Any} or Function
+
+type Solver
+    name :: ASCIIString
+    time :: Real
+    iteration :: Int
+    problems :: Vector{Union{FieldProblem, BoundaryProblem}}
+    is_linear_system :: Bool
+    nonlinear_system_max_iterations :: Int64
+    nonlinear_system_convergence_tolerance :: Float64
+    linear_system_solver :: Symbol
+end
+
+function Solver(name::ASCIIString="default solver", time::Real=0.0)
+    return Solver(
+        name,  # name
+        time,  # time
+        0,     # iteration counter
+        [],    # array of problems
+        false, # is this a linear system which can be solved in a single iteration?
+        10,    # max nonlinear iterations
+        5.0e-5, # nonlinear iteration convergence tolerance
+        :DirectLinearSolver # linear system solution method
+        )
+end
+
+function push!(solver::Solver, problem::Union{FieldProblem, BoundaryProblem})
+    push!(solver.problems, problem)
+end
+
+# one-liner helpers to identify problem types
+
+function is_field_problem(problem)
+    return typeof(problem) <: FieldProblem
+end
+
+function is_boundary_problem(problem)
+    return typeof(problem) <: BoundaryProblem
+end
+
+function is_dirichlet_problem(problem)
+    return typeof(problem) <: Union{BoundaryProblem{DirichletProblem}, BoundaryProblem{DirichletProblem{DualBasis}}}
+end
+
+function is_mortar_problem(problem)
+    return typeof(problem) <: BoundaryProblem{MortarProblem}
+end
+
+function get_field_problems(solver::Solver)
+    filter(is_field_problem, solver.problems)
+end
+
+function get_boundary_problems(solver::Solver)
+    filter(is_boundary_problem, solver.problems)
+end
+
+function get_dirichlet_problems(solver::Solver)
+    filter(is_dirichlet_problem, solver.problems)
+end
+
+function get_mortar_problems(solver::Solver)
+    filter(is_mortar_problem, solver.problems)
+end
+
+"""Return one combined field assembly for a set of field problems.
+
+Parameters
+----------
+solver :: Solver
+
+Returns
+-------
+K, f :: SparseMatrixCOO
+
+Notes
+-----
+If several field problems exists, they are simply summed together, so
+problems must have unique node ids.
+
+"""
+function get_field_assembly(solver::Solver)
+    return get_field_assembly(get_field_problems(solver))
+end
+function get_field_assembly(problems::Vector{Union{BoundaryProblem, FieldProblem}})
+    K = SparseMatrixCOO()
+    f = SparseMatrixCOO()
+    for problem in problems
+        append!(K, problem.assembly.stiffness_matrix)
+        append!(f, problem.assembly.force_vector)
+    end
+    return K, f
+end
+
+""" Return one combined boundary assembly for a set of boundary problems.
+
+Returns
+-------
+C1, C2, D, g :: SparseMatrixCOO
+
+"""
+function get_boundary_assembly(solver::Solver)
+    return get_boundary_assembly(get_boundary_problems(solver))
+end
+function get_boundary_assembly(problems::Vector{Union{BoundaryProblem, FieldProblem}})
+    C1 = SparseMatrixCOO()
+    C2 = SparseMatrixCOO()
+    D = SparseMatrixCOO()
+    g = SparseMatrixCOO()
+    for problem in problems
+        append!(C1, problem.assembly.C1)
+        append!(C2, problem.assembly.C2)
+        append!(D, problem.assembly.D)
+        append!(g, problem.assembly.g)
+    end
+    return C1, C2, D, g
+end
+
+""" Solve linear system using LU factorization (UMFPACK).
+"""
+function solve_linear_system!(solver::Solver, ::Type{Val{:DirectLinearSolver}})
+    info("solving linear system of $(length(solver.problems)) problems.")
+    t0 = time()
+
+    # assemble field problems
+    K, f = get_field_assembly(solver)
+    K = sparse(K)
+    dim = size(K, 1)
+    f = sparse(f, dim, 1)
+
+    # assemble boundary problems
+    C1, C2, D, g = get_boundary_assembly(solver)
+    C1 = sparse(C1, dim, dim)
+    C2 = sparse(C2, dim, dim)
+    D = sparse(D, dim, dim)
+    g = sparse(g, dim, 1)
+
+    # construct global system Ax=b and solve using lu factorization
+    A = [K C1'; C2 D]
+    b = [f; g]
+    nz1 = sort(unique(rowvals(A)))
+    nz2 = sort(unique(rowvals(A')))
+    x = zeros(length(b))
+    x[nz1] = lufact(A[nz1,nz2]) \ full(b[nz1])
+
+    # update solutions
+    u = x[1:dim]
+    la = x[dim+1:end]
+    for problem in solver.problems
+        typeof(problem) <: FieldProblem && update!(problem, u)
+        typeof(problem) <: BoundaryProblem && update!(problem, la)
+    end
+
+    info("UMFPACK: solved in ", time()-t0, " seconds. norm = ", norm(u))
+end
+
+""" Check convergence of problems.
+
+Notes
+-----
+Default convergence criteria is obtained by checking each sub-problem convergence.
+"""
+function has_converged(solver::Solver; print_convergence_information=true)
+    converged = true
+    for problem in solver.problems
+        has_converged = problem.assembly.solution_norm_change < solver.nonlinear_system_convergence_tolerance
+        if print_convergence_information
+            @printf "% 30s | %8.3f | %s\n" problem.name problem.assembly.solution_norm_change has_converged
+        end
+        converged &= has_converged
+    end
+    return converged || solver.is_linear_system
+end
+
+type NonlinearConvergenceError <: Exception
+    solver :: Solver
+end
+
+function Base.showerror(io::IO, exception::NonlinearConvergenceError)
+    max_iters = exception.solver.nonlinear_system_max_iterations
+    print(io, "nonlinear iteration did not converge in $max_iters iterations!")
+end
+
+""" Main solver loop.
+"""
+function call(solver::Solver)
+    # 1. initialize each problem so that we can start nonlinear iterations
+    for problem in solver.problems
+        initialize!(problem, solver.time)
+    end
+
+    # 2. start non-linear iterations
+    for solver.iteration=1:solver.nonlinear_system_max_iterations
+        # 2.1 update linearized assemblies (if needed)
+        for problem in solver.problems
+            problem.assembly.changed = true  # force reassembly
+            assemble!(problem, solver.time)
+        end
+
+        # 2.2 call solver for linearized system (default: direct lu factorization)
+        solve_linear_system!(solver, Val{solver.linear_system_solver})
+
+        # 2.3 update solution back to elements
+        for problem in solver.problems
+            update!(problem, problem.assembly.solution, Val{:elements})
+        end
+
+        # 2.4 check convergence
+        if has_converged(solver)
+            info("Converged in $(solver.iteration) iterations.")
+            return true
+        end
+    end
+
+    # 3. did not converge
+    throw(NonlinearConvergenceError(solver))
+end
+