diff --git a/gtsam/nonlinear/NonlinearConjugateGradientOptimizer.h b/gtsam/nonlinear/NonlinearConjugateGradientOptimizer.h
index abf6b257a..a85f27425 100644
--- a/gtsam/nonlinear/NonlinearConjugateGradientOptimizer.h
+++ b/gtsam/nonlinear/NonlinearConjugateGradientOptimizer.h
@@ -200,6 +200,10 @@ boost::tuple<V, int> nonlinearConjugateGradient(const S &system,
     currentValues = system.advance(prevValues, alpha, direction);
     currentError = system.error(currentValues);
 
+    // User hook:
+    if (params.iterationHook)
+      params.iterationHook(iteration, prevError, currentError);
+
     // Maybe show output
     if (params.verbosity >= NonlinearOptimizerParams::ERROR)
       std::cout << "iteration: " << iteration << ", currentError: " << currentError << std::endl;
diff --git a/gtsam/nonlinear/NonlinearOptimizer.cpp b/gtsam/nonlinear/NonlinearOptimizer.cpp
index fd9961742..0d7e9e17f 100644
--- a/gtsam/nonlinear/NonlinearOptimizer.cpp
+++ b/gtsam/nonlinear/NonlinearOptimizer.cpp
@@ -97,7 +97,11 @@ void NonlinearOptimizer::defaultOptimize() {
 
     // Update newError for either printouts or conditional-end checks:
     newError = error();
-    
+
+    // User hook:
+    if (params.iterationHook)
+      params.iterationHook(iterations(), currentError, newError);
+
     // Maybe show output
     if (params.verbosity >= NonlinearOptimizerParams::VALUES)
       values().print("newValues");
diff --git a/gtsam/nonlinear/NonlinearOptimizer.h b/gtsam/nonlinear/NonlinearOptimizer.h
index 9935f44ce..6fe369dd3 100644
--- a/gtsam/nonlinear/NonlinearOptimizer.h
+++ b/gtsam/nonlinear/NonlinearOptimizer.h
@@ -81,7 +81,7 @@ protected:
 
 public:
   /** A shared pointer to this class */
-  typedef boost::shared_ptr<const NonlinearOptimizer> shared_ptr;
+  using shared_ptr = boost::shared_ptr<const NonlinearOptimizer>;
 
   /// @name Standard interface
   /// @{
diff --git a/gtsam/nonlinear/NonlinearOptimizerParams.h b/gtsam/nonlinear/NonlinearOptimizerParams.h
index 65fdd1c92..53805f5f0 100644
--- a/gtsam/nonlinear/NonlinearOptimizerParams.h
+++ b/gtsam/nonlinear/NonlinearOptimizerParams.h
@@ -33,22 +33,19 @@ namespace gtsam {
  */
 class GTSAM_EXPORT NonlinearOptimizerParams {
 public:
+  NonlinearOptimizerParams() = default;
+
   /** See NonlinearOptimizerParams::verbosity */
   enum Verbosity {
     SILENT, TERMINATION, ERROR, VALUES, DELTA, LINEAR
   };
 
-  size_t maxIterations; ///< The maximum iterations to stop iterating (default 100)
-  double relativeErrorTol; ///< The maximum relative error decrease to stop iterating (default 1e-5)
-  double absoluteErrorTol; ///< The maximum absolute error decrease to stop iterating (default 1e-5)
-  double errorTol; ///< The maximum total error to stop iterating (default 0.0)
-  Verbosity verbosity; ///< The printing verbosity during optimization (default SILENT)
-  Ordering::OrderingType orderingType; ///< The method of ordering use during variable elimination (default COLAMD)
-
-  NonlinearOptimizerParams() :
-      maxIterations(100), relativeErrorTol(1e-5), absoluteErrorTol(1e-5), errorTol(
-          0.0), verbosity(SILENT), orderingType(Ordering::COLAMD),
-          linearSolverType(MULTIFRONTAL_CHOLESKY) {}
+  size_t maxIterations = 100; ///< The maximum iterations to stop iterating (default 100)
+  double relativeErrorTol = 1e-5; ///< The maximum relative error decrease to stop iterating (default 1e-5)
+  double absoluteErrorTol = 1e-5; ///< The maximum absolute error decrease to stop iterating (default 1e-5)
+  double errorTol = 0.0; ///< The maximum total error to stop iterating (default 0.0)
+  Verbosity verbosity = SILENT; ///< The printing verbosity during optimization (default SILENT)
+  Ordering::OrderingType orderingType = Ordering::COLAMD; ///< The method of ordering use during variable elimination (default COLAMD)
 
   virtual ~NonlinearOptimizerParams() {
   }
@@ -71,6 +68,15 @@ public:
   static Verbosity verbosityTranslator(const std::string &s) ;
   static std::string verbosityTranslator(Verbosity value) ;
 
+  /** Type for an optional user-provided hook to be called after each 
+   * internal optimizer iteration */
+  using IterationHook = std::function<
+    void(size_t /*iteration*/, double/*errorBefore*/, double/*errorAfter*/)>;
+
+  /** Optional user-provided iteration hook to be called after each 
+   * optimization iteration (Default: empty) */
+  IterationHook iterationHook;
+
   /** See NonlinearOptimizerParams::linearSolverType */
   enum LinearSolverType {
     MULTIFRONTAL_CHOLESKY,
@@ -81,7 +87,7 @@ public:
     CHOLMOD, /* Experimental Flag */
   };
 
-  LinearSolverType linearSolverType; ///< The type of linear solver to use in the nonlinear optimizer
+  LinearSolverType linearSolverType = MULTIFRONTAL_CHOLESKY; ///< The type of linear solver to use in the nonlinear optimizer
   boost::optional<Ordering> ordering; ///< The optional variable elimination ordering, or empty to use COLAMD (default: empty)
   IterativeOptimizationParameters::shared_ptr iterativeParams; ///< The container for iterativeOptimization parameters. used in CG Solvers.
 
diff --git a/tests/testNonlinearOptimizer.cpp b/tests/testNonlinearOptimizer.cpp
index dc19801a2..6415174d5 100644
--- a/tests/testNonlinearOptimizer.cpp
+++ b/tests/testNonlinearOptimizer.cpp
@@ -566,6 +566,58 @@ TEST( NonlinearOptimizer, logfile )
 //  EXPECT(actual.str()==expected.str());
 }
 
+/* ************************************************************************* */
+TEST( NonlinearOptimizer, iterationHook_LM )
+{
+  NonlinearFactorGraph fg(example::createReallyNonlinearFactorGraph());
+
+  Point2 x0(3,3);
+  Values c0;
+  c0.insert(X(1), x0);
+
+  // Levenberg-Marquardt
+  LevenbergMarquardtParams lmParams;
+  size_t lastIterCalled = 0;
+  lmParams.iterationHook = [&](size_t iteration, double oldError, double newError)
+  {
+    // Tests:
+    lastIterCalled = iteration;
+    EXPECT(newError<oldError);
+    
+    // Example of evolution printout:
+    //std::cout << "iter: " << iteration << " error: " << oldError << " => " << newError <<"\n";
+  };
+  LevenbergMarquardtOptimizer(fg, c0, lmParams).optimize();
+  
+  EXPECT(lastIterCalled>5);
+}
+/* ************************************************************************* */
+TEST( NonlinearOptimizer, iterationHook_CG )
+{
+  NonlinearFactorGraph fg(example::createReallyNonlinearFactorGraph());
+
+  Point2 x0(3,3);
+  Values c0;
+  c0.insert(X(1), x0);
+
+  // Levenberg-Marquardt
+  NonlinearConjugateGradientOptimizer::Parameters cgParams;
+  size_t lastIterCalled = 0;
+  cgParams.iterationHook = [&](size_t iteration, double oldError, double newError)
+  {
+    // Tests:
+    lastIterCalled = iteration;
+    EXPECT(newError<oldError);
+    
+    // Example of evolution printout:
+    //std::cout << "iter: " << iteration << " error: " << oldError << " => " << newError <<"\n";
+  };
+  NonlinearConjugateGradientOptimizer(fg, c0, cgParams).optimize();
+  
+  EXPECT(lastIterCalled>5);
+}
+
+
 /* ************************************************************************* */
 //// Minimal traits example
 struct MyType : public Vector3 {