diff --git a/src/opt/opt_context.cpp b/src/opt/opt_context.cpp
index 095c8f1f6..754b2adb2 100644
--- a/src/opt/opt_context.cpp
+++ b/src/opt/opt_context.cpp
@@ -78,7 +78,7 @@ namespace opt {
             // SASSERT(instanceof(*s, opt_solver));
             // if (!instsanceof ...) { throw ... invalid usage ..} 
             is_sat = optimize_objectives(dynamic_cast<opt_solver&>(*s), m_objectives, values);
-            std::cout << "is-sat: " << is_sat << "\n";
+            std::cout << "is-sat: " << is_sat << std::endl;
 
             if (is_sat != l_true) {
                 return;
@@ -88,15 +88,14 @@ namespace opt {
                 if (!m_is_max[i]) {
                     values[i].neg();
                 }
-                std::cout << "objective function: " << mk_pp(m_objectives[i].get(), m) << " -> " << values[i].to_string() << "\n";                
+                std::cout << "objective value: " << mk_pp(m_objectives[i].get(), m) << " -> " << values[i].to_string() << std::endl;                
             }
         }     
 
         if (m_objectives.empty() && m_soft_constraints.empty()) {
             is_sat = s->check_sat(0,0);
-            std::cout << "nothing to optimize: is-sat " << is_sat << "\n";
+            std::cout << "nothing to optimize: is-sat " << is_sat << std::endl;
         }
-
     }
         
     bool context::is_maxsat_problem() const {
diff --git a/src/opt/opt_solver.cpp b/src/opt/opt_solver.cpp
index edeaf0441..6cf7c59ed 100644
--- a/src/opt/opt_solver.cpp
+++ b/src/opt/opt_solver.cpp
@@ -2,6 +2,7 @@
 #include"opt_solver.h"
 #include"smt_context.h"
 #include"theory_arith.h"
+#include"theory_diff_logic.h"
 
 namespace opt {
 
@@ -55,6 +56,12 @@ namespace opt {
         else if (typeid(smt::theory_i_arith) == typeid(*arith_theory)) {   
             return dynamic_cast<smt::theory_i_arith&>(*arith_theory); 
         }
+        else if (typeid(smt::theory_rdl&) == typeid(*arith_theory)) {   
+            return dynamic_cast<smt::theory_rdl&>(*arith_theory); 
+        }
+        else if (typeid(smt::theory_idl&) == typeid(*arith_theory)) {   
+            return dynamic_cast<smt::theory_idl&>(*arith_theory); 
+        }
         else {
             UNREACHABLE();
             return dynamic_cast<smt::theory_mi_arith&>(*arith_theory); 
diff --git a/src/smt/theory_diff_logic.h b/src/smt/theory_diff_logic.h
index 7302ccfd4..579195c84 100644
--- a/src/smt/theory_diff_logic.h
+++ b/src/smt/theory_diff_logic.h
@@ -37,7 +37,7 @@ Revision History:
 #include"smt_model_generator.h"
 #include"numeral_factory.h"
 #include"smt_clause.h"
-
+#include"theory_opt.h"
 
 // The DL theory can represent term such as n + k, where n is an enode and k is a numeral.
 namespace smt {
@@ -59,7 +59,7 @@ namespace smt {
     };
 
     template<typename Ext>
-    class theory_diff_logic : public theory, private Ext {
+    class theory_diff_logic : public theory, public theory_opt, private Ext {
         
         typedef typename Ext::numeral numeral;
 
@@ -296,6 +296,18 @@ namespace smt {
         
         virtual void collect_statistics(::statistics & st) const;
 
+        
+        // -----------------------------------
+        //
+        // Optimization
+        //
+        // -----------------------------------
+
+        virtual bool maximize(theory_var v);
+        virtual theory_var add_objective(app* term);
+        virtual inf_eps_rational<inf_rational> get_objective_value(theory_var v);
+        inf_rational m_objective;      
+
     private:        
 
         virtual void new_eq_eh(theory_var v1, theory_var v2, justification& j);
@@ -374,6 +386,53 @@ namespace smt {
         srdl_ext() : m_epsilon(s_integer(0),true) {}
     };
 
+    // Solve minimum cost flow problem using Network Simplex algorithm
+    class network_simplex {
+        svector<dl_var>  m_nodes;
+        svector<edge_id> m_edges;
+        // Denote costs c_ij on edge (i, j)
+        vector<rational> m_costs;
+        // Denote supply/demand b_i on node i
+        vector<rational> m_capacities;
+
+        // Keep optimal solution of the min cost flow problem
+        inf_rational m_objective;
+
+    public:
+        // Create a spanning tree using Kruskal algorithm
+        virtual svector<edge_id> & create_spanning_tree();
+            
+        // A spanning tree is a basis in network simplex.
+        // Check whether the edges' associated costs could be reduced
+        virtual rational calculate_reduced_costs(svector<edge_id> & spanning_tree);
+
+        // If all reduced costs are non-negative, the current flow is optimal
+        virtual bool is_optimal(svector<edge_id> & spanning_tree);
+
+        // Choose an edge with negative reduced cost
+        virtual edge_id choose_entering_edge();
+
+        // Send as much flow as possible around the cycle, the first basic edge with flow 0 will leave
+        edge_id choose_leaving_edge();
+
+        virtual bool is_unbounded();
+
+        // Minimize cost flows
+        // Return true if found an optimal solution, and return false if unbounded
+        virtual bool minimize();
+    };
+
+    /* Notes:
+
+        We need a function to reduce DL constraints to min cost flow problem 
+        and another function to convert from min cost flow solution to DL solution.
+
+        It remains unclear how to convert DL assignment to a basic feasible solution of Network Simplex.
+        A naive approach is to run Kruskal in order to get a spanning tree.
+
+        The network_simplex class hasn't had multiple pivoting strategies yet.
+    */
+
 
     typedef theory_diff_logic<idl_ext>  theory_idl;
     typedef theory_diff_logic<sidl_ext> theory_fidl;
diff --git a/src/smt/theory_diff_logic_def.h b/src/smt/theory_diff_logic_def.h
index 390803957..60505146d 100644
--- a/src/smt/theory_diff_logic_def.h
+++ b/src/smt/theory_diff_logic_def.h
@@ -996,5 +996,23 @@ void theory_diff_logic<Ext>::get_implied_bound_antecedents(edge_id bridge_edge,
     m_graph.explain_subsumed_lazy(bridge_edge, subsumed_edge, f);
 }
 
+template<typename Ext>
+bool theory_diff_logic<Ext>::maximize(theory_var v) {
+    NOT_IMPLEMENTED_YET();
+    return false;
+}
+
+template<typename Ext>
+theory_var theory_diff_logic<Ext>::add_objective(app* term) {
+    // Internalizing may not succeed since objective can be LRA
+    return null_theory_var; 
+}
+
+template<typename Ext>
+inf_eps_rational<inf_rational> theory_diff_logic<Ext>::get_objective_value(theory_var v) { 
+    inf_eps_rational<inf_rational> val(m_objective);     
+    return val; 
+}
+
 #endif /* _THEORY_DIFF_LOGIC_DEF_H_ */