diff --git a/gtsam/nonlinear/NonlinearConjugateGradientOptimizer.h b/gtsam/nonlinear/NonlinearConjugateGradientOptimizer.h
index 04d4734a4..7829df98e 100644
--- a/gtsam/nonlinear/NonlinearConjugateGradientOptimizer.h
+++ b/gtsam/nonlinear/NonlinearConjugateGradientOptimizer.h
@@ -156,7 +156,7 @@ boost::tuple<V, int> nonlinearConjugateGradient(const S &system,
 
   // GTSAM_CONCEPT_MANIFOLD_TYPE(V);
 
-  int iteration = 0;
+  size_t iteration = 0;
 
   // check if we're already close enough
   double currentError = system.error(initial);
diff --git a/gtsam/nonlinear/NonlinearOptimizer.h b/gtsam/nonlinear/NonlinearOptimizer.h
index 7b9fdba41..d67d51338 100644
--- a/gtsam/nonlinear/NonlinearOptimizer.h
+++ b/gtsam/nonlinear/NonlinearOptimizer.h
@@ -42,7 +42,7 @@ public:
   double error;
 
   /** The number of optimization iterations performed. */
-  int iterations;
+  size_t iterations;
 
   NonlinearOptimizerState() {}
 
@@ -50,7 +50,7 @@ public:
   virtual ~NonlinearOptimizerState() {}
 
 protected:
-  NonlinearOptimizerState(const NonlinearFactorGraph& graph, const Values& values, unsigned int iterations = 0) :
+  NonlinearOptimizerState(const NonlinearFactorGraph& graph, const Values& values, size_t iterations = 0) :
     values(values), error(graph.error(values)), iterations(iterations) {}
 
   NonlinearOptimizerState(const Values& values, double error, unsigned int iterations) :
@@ -166,7 +166,7 @@ public:
   double error() const { return _state().error; }
 
   /// return number of iterations
-  int iterations() const { return _state().iterations; }
+  size_t iterations() const { return _state().iterations; }
 
   /// return values
   const Values& values() const { return _state().values; }
diff --git a/gtsam/nonlinear/NonlinearOptimizerParams.h b/gtsam/nonlinear/NonlinearOptimizerParams.h
index 10de6994f..93c45a8c4 100644
--- a/gtsam/nonlinear/NonlinearOptimizerParams.h
+++ b/gtsam/nonlinear/NonlinearOptimizerParams.h
@@ -38,7 +38,7 @@ public:
     SILENT, TERMINATION, ERROR, VALUES, DELTA, LINEAR
   };
 
-  int maxIterations; ///< The maximum iterations to stop iterating (default 100)
+  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)
@@ -54,7 +54,7 @@ public:
   }
   virtual void print(const std::string& str = "") const;
 
-  int getMaxIterations() const {
+  size_t getMaxIterations() const {
     return maxIterations;
   }
   double getRelativeErrorTol() const {