diff --git a/.gitignore b/.gitignore
index 4d2cd84a85..4c81ab67b7 100644
--- a/.gitignore
+++ b/.gitignore
@@ -9,6 +9,7 @@
 tmp
 # Ignore build trees.
 /build*
+/out
 # CLion integrated development environment
 /cmake-build-*
 # Ignore install dir.
@@ -28,3 +29,5 @@ dependencies/*
 # MAC File System files
 .DS_Store
 
+/CMakeSettings.json
+
diff --git a/Applications/opensense/opensense.cpp b/Applications/opensense/opensense.cpp
index 15f6a40e3f..0f2fb833ff 100644
--- a/Applications/opensense/opensense.cpp
+++ b/Applications/opensense/opensense.cpp
@@ -453,7 +453,7 @@ void addImuFramesFromMarkers(const string& modelFile, const string& markersFile)
     // store joint initial pose from marker IK as default pose for the model. 
     model.setPropertiesFromState(s);
 
-    for (int i = 0; i < offsets.size(); ++i) {
+    for (int i = 0; i < (int)offsets.size(); ++i) {
         // add imu offset frames to the model with model taking ownership
         bodies[i]->addComponent(offsets[i]);
     }
diff --git a/CMakeLists.txt b/CMakeLists.txt
index e889020c87..a4c5ebbf59 100644
--- a/CMakeLists.txt
+++ b/CMakeLists.txt
@@ -359,8 +359,6 @@ if(WIN32 AND ${CMAKE_C_COMPILER} MATCHES "cl")
     set(mxcpu ${BUILD_LIMIT_PARALLEL_COMPILES}) # abbreviation
 
     ## C++
-    # Disable "C4068: unknown pragma" warning.
-    set(CMAKE_CXX_FLAGS "${CMAKE_CXX_FLAGS} /wd4068") 
     set(CMAKE_CXX_FLAGS_DEBUG
     "/MP${mxcpu} /D _DEBUG /MDd /Od /Ob0 /RTC1 /Zi /bigobj /GS- ${CL_INST_SET}"
         CACHE STRING "VC++ Debug build compile flags" FORCE)
@@ -388,6 +386,13 @@ if(WIN32 AND ${CMAKE_C_COMPILER} MATCHES "cl")
     "/MP${mxcpu} /D NDEBUG /MD  /O1 /Ob1 /Oi /GS- ${CL_INST_SET}"
         CACHE STRING "VC++ MinSizeRel build compile flags" FORCE)
 
+    # Treat warnings as errors.
+    # Ideally we would increase the warning level to 4 but that generates
+    # warnings in our dependencies.
+    # Convert warning 4388 ('operator' signed/unsigned mismatch) from
+    # level 4 to level 3 for consistency with Clang.
+    add_compile_options(/WX /w34388) # /W4
+
 endif()
 
 set(BUILD_JAVA_WRAPPING OFF CACHE BOOL "Build Java wrapping (needed if you're building the GUI or the Matlab wrapping; requires that you have SWIG and Java installed on your machine.)")
diff --git a/OpenSim/Auxiliary/auxiliaryTestFunctions.h b/OpenSim/Auxiliary/auxiliaryTestFunctions.h
index 2fe01fa163..df97fdda6c 100644
--- a/OpenSim/Auxiliary/auxiliaryTestFunctions.h
+++ b/OpenSim/Auxiliary/auxiliaryTestFunctions.h
@@ -133,7 +133,7 @@ void ASSERT_EQUAL( const Container& vecA,
         throw OpenSim::Exception(message, file, line);
     }
     else {
-        for (auto i = 0; i < vecA.size(); ++i) {
+        for (int i = 0; i < (int)vecA.size(); ++i) {
             // if both values are NaN treat them as being equivalent
             if ( SimTK::isNaN(vecA[i]) && SimTK::isNaN(vecB[i]) )
                 continue;
diff --git a/OpenSim/Common/APDMDataReader.cpp b/OpenSim/Common/APDMDataReader.cpp
index 933478b3d4..6cfeb0681a 100644
--- a/OpenSim/Common/APDMDataReader.cpp
+++ b/OpenSim/Common/APDMDataReader.cpp
@@ -226,7 +226,7 @@ void APDMDataReader::find_start_column(std::vector<std::string> tokens,
         found_index = static_cast<int>(std::distance(tokens.begin(), it));
         // now check the following indices for match with remaining search_labels 
         bool match = true;
-        for (int remaining = 1; remaining < search_labels.size() && match; remaining++) {
+        for (int remaining = 1; remaining < (int)search_labels.size() && match; remaining++) {
             match = tokens[found_index + remaining].
                 compare(sensorName + search_labels[remaining]) == 0;
         }
diff --git a/OpenSim/Common/ComponentSocket.cpp b/OpenSim/Common/ComponentSocket.cpp
index 2415af7bc5..7feb0767b2 100644
--- a/OpenSim/Common/ComponentSocket.cpp
+++ b/OpenSim/Common/ComponentSocket.cpp
@@ -45,7 +45,7 @@ void AbstractSocket::prependComponentPathToConnecteePath(
         ComponentPath path(getConnecteePath(iConn));
         if (path.isAbsolute()) {
             ComponentPath newPath(pathToPrepend);
-            for (int iPath = 0; iPath < path.getNumPathLevels();
+            for (int iPath = 0; iPath < (int)path.getNumPathLevels();
                  ++iPath) {
                 newPath.pushBack(
                         path.getSubcomponentNameAtLevel(iPath));
@@ -71,7 +71,7 @@ void AbstractInput::prependComponentPathToConnecteePath(
         ComponentPath path(componentPath);
         if (path.isAbsolute()) {
             ComponentPath newPath(pathToPrepend);
-            for (int iPath = 0; iPath < path.getNumPathLevels();
+            for (int iPath = 0; iPath < (int)path.getNumPathLevels();
                  ++iPath) {
                 newPath.pushBack(
                         path.getSubcomponentNameAtLevel(iPath));
diff --git a/OpenSim/Common/DataTable.h b/OpenSim/Common/DataTable.h
index 6ee78c3385..27985b9a3d 100644
--- a/OpenSim/Common/DataTable.h
+++ b/OpenSim/Common/DataTable.h
@@ -938,7 +938,7 @@ class DataTable_ : public AbstractDataTable {
                          InvalidArgument,
                          "Column-label '" + columnLabel + "' already exists in "
                          "the DataTable.");
-        OPENSIM_THROW_IF(depCol.nrow() != getNumRows(),
+        OPENSIM_THROW_IF(depCol.nrow() != (int)getNumRows(),
                          IncorrectNumRows,
                          static_cast<size_t>(getNumRows()),
                          static_cast<size_t>(depCol.nrow()));
@@ -1223,10 +1223,10 @@ class DataTable_ : public AbstractDataTable {
     DataTable_(const std::vector<ETX>& indVec,
         const SimTK::Matrix_<ETY>& depData,
         const std::vector<std::string>& labels) {
-        OPENSIM_THROW_IF(indVec.size() != depData.nrow(), InvalidArgument,
+        OPENSIM_THROW_IF((int)indVec.size() != depData.nrow(), InvalidArgument,
             "Length of independent column does not match number of rows of "
             "dependent data.");
-        OPENSIM_THROW_IF(labels.size() != depData.ncol(), InvalidArgument,
+        OPENSIM_THROW_IF((int)labels.size() != depData.ncol(), InvalidArgument,
             "Number of labels does not match number of columns of "
             "dependent data.");
 
diff --git a/OpenSim/Common/DelimFileAdapter.h b/OpenSim/Common/DelimFileAdapter.h
index 21f4814514..adaa500042 100644
--- a/OpenSim/Common/DelimFileAdapter.h
+++ b/OpenSim/Common/DelimFileAdapter.h
@@ -433,7 +433,7 @@ DelimFileAdapter<T>::extendRead(const std::string& fileName) const {
 
         auto row_vector = readElems(row);
 
-        OPENSIM_THROW_IF(row_vector.size() != column_labels.size(),
+        OPENSIM_THROW_IF(row_vector.size() != (int)column_labels.size(),
             RowLengthMismatch,
             fileName,
             line_num,
diff --git a/OpenSim/Common/Test/testAPDMDataReader.cpp b/OpenSim/Common/Test/testAPDMDataReader.cpp
index 3aa79102be..caa2270423 100644
--- a/OpenSim/Common/Test/testAPDMDataReader.cpp
+++ b/OpenSim/Common/Test/testAPDMDataReader.cpp
@@ -36,7 +36,7 @@ int main() {
         std::vector<std::string> imu_names{ "torso", "pelvis", "shank" };
         std::vector<std::string> names_in_experiment{ "Static", "Upper", "Middle" };
         // Programmatically add items to name mapping, write to xml
-        for (int index = 0; index < imu_names.size(); ++index) {
+        for (int index = 0; index < (int)imu_names.size(); ++index) {
             ExperimentalSensor  nextSensor(names_in_experiment[index], imu_names[index]);
             readerSettings.append_ExperimentalSensors(nextSensor);
         }
@@ -113,7 +113,7 @@ void testAPDMFormat7() {
     std::vector<std::string> imu_names{ "imu1", "imu2", "imu4" };
     std::vector<std::string> names_in_experiment{ "Lumbar", "Right Ankle", "Right Foot" };
     // Programmatically add items to name mapping, write to xml
-    for (int index = 0; index < imu_names.size(); ++index) {
+    for (int index = 0; index < (int)imu_names.size(); ++index) {
         ExperimentalSensor  nextSensor(names_in_experiment[index], imu_names[index]);
         readerSettings.append_ExperimentalSensors(nextSensor);
     }
diff --git a/OpenSim/Common/Test/testXsensDataReader.cpp b/OpenSim/Common/Test/testXsensDataReader.cpp
index 80b682e7a9..dc020672d7 100644
--- a/OpenSim/Common/Test/testXsensDataReader.cpp
+++ b/OpenSim/Common/Test/testXsensDataReader.cpp
@@ -42,7 +42,7 @@ int main() {
         std::vector<std::string> imu_names{ "shank", "thigh" };
         std::vector<std::string> file_names{ "000_00B421AF", "000_00B4227B" };
         // Programmatically add items to Map, write to xml
-        for (int index = 0; index < imu_names.size(); ++index) {
+        for (int index = 0; index < (int)imu_names.size(); ++index) {
             ExperimentalSensor  nextSensor(file_names[index], imu_names[index]);
             readerSettings.append_ExperimentalSensors(nextSensor);
         }
diff --git a/OpenSim/Simulation/Model/ExternalLoads.cpp b/OpenSim/Simulation/Model/ExternalLoads.cpp
index 457d65d41e..59e29512ed 100644
--- a/OpenSim/Simulation/Model/ExternalLoads.cpp
+++ b/OpenSim/Simulation/Model/ExternalLoads.cpp
@@ -251,7 +251,7 @@ void ExternalLoads::transformPointsExpressedInGroundToAppliedBodies(
     }
     // Once we've transformed the forces (done with computation),
     // then replace them in the Set
-    for (int i = 0; i < transformedForces.size(); ++i) {
+    for (int i = 0; i < (int)transformedForces.size(); ++i) {
         ExternalForce *transformedExf = transformedForces[i];
         if (transformedExf) {
             set(i, transformedExf);
diff --git a/OpenSim/Simulation/OpenSense/InverseKinematicsStudy.cpp b/OpenSim/Simulation/OpenSense/InverseKinematicsStudy.cpp
index 72d611f272..ff57259cd8 100644
--- a/OpenSim/Simulation/OpenSense/InverseKinematicsStudy.cpp
+++ b/OpenSim/Simulation/OpenSense/InverseKinematicsStudy.cpp
@@ -96,7 +96,7 @@ void InverseKinematicsStudy::
     std::cout << "press any key to visualize experimental marker data ..." << std::endl;
     std::cin >> c;
 
-    for (size_t j =startEnd[0]; j <= startEnd[1]; j=j+10) {
+    for (int j =startEnd[0]; j <= startEnd[1]; j=j+10) {
         std::cout << "time: " << times[j] << "s" << std::endl;
         state.setTime(times[j]);
         previewWorld.realizePosition(state);
diff --git a/OpenSim/Simulation/OpenSense/OpenSenseUtilities.cpp b/OpenSim/Simulation/OpenSense/OpenSenseUtilities.cpp
index 571a29d971..eb113c3291 100644
--- a/OpenSim/Simulation/OpenSense/OpenSenseUtilities.cpp
+++ b/OpenSim/Simulation/OpenSense/OpenSenseUtilities.cpp
@@ -52,7 +52,7 @@ TimeSeriesTable_<SimTK::Rotation> OpenSenseUtilities::
     SimTK::Matrix_<SimTK::Rotation> matrix(int(nt), nc, Rotation());
 
     int cnt = 0;
-    for (size_t i = startEnd[0]; i <= startEnd[1]; ++i) {
+    for (int i = startEnd[0]; i <= startEnd[1]; ++i) {
         newTimes[cnt] = times[i];
         const auto& quatRow = quaternionsTable.getRowAtIndex(i);
         for (int j = 0; j < nc; ++j) {
diff --git a/OpenSim/Simulation/SimulationUtilities.cpp b/OpenSim/Simulation/SimulationUtilities.cpp
index d127c619c9..3283ab24f0 100644
--- a/OpenSim/Simulation/SimulationUtilities.cpp
+++ b/OpenSim/Simulation/SimulationUtilities.cpp
@@ -181,7 +181,7 @@ void OpenSim::updateSocketConnecteesBySearch(Model& model)
     int numSocketsUpdated = 0;
     for (auto& comp : model.updComponentList()) {
         const auto socketNames = comp.getSocketNames();
-        for (int i = 0; i < socketNames.size(); ++i) {
+        for (int i = 0; i < (int)socketNames.size(); ++i) {
             auto& socket = comp.updSocket(socketNames[i]);
             try {
                 socket.finalizeConnection(model);
diff --git a/appveyor.yml b/appveyor.yml
index 9066bb6e77..8ea886d136 100644
--- a/appveyor.yml
+++ b/appveyor.yml
@@ -102,12 +102,11 @@ build_script:
   # for some reason.
   - cd %OPENSIM_BUILD_DIR%
   # Configure. # TODO -DBUILD_SIMM_TRANSLATOR=ON
-  # Set the CXXFLAGS environment variable to turn warnings into errors.
   # Skip timing test section included by default
-  - cmake -E env CXXFLAGS="/WX -DSKIP_TIMING" cmake %OPENSIM_SOURCE_DIR% -G"%CMAKE_GENERATOR%" -T"%CMAKE_TOOLSET%" -DSIMBODY_HOME=C:\simbody%NUGET_PACKAGE_ID_SUFFIX% -DOPENSIM_DEPENDENCIES_DIR=%OPENSIM_DEPENDENCIES_INSTALL_DIR% -DCMAKE_INSTALL_PREFIX=%OPENSIM_INSTALL_DIR% -DBUILD_JAVA_WRAPPING=ON -DBUILD_PYTHON_WRAPPING=ON -DWITH_BTK:BOOL=ON
+  - cmake -E env CXXFLAGS="-DSKIP_TIMING" cmake %OPENSIM_SOURCE_DIR% -G"%CMAKE_GENERATOR%" -T"%CMAKE_TOOLSET%" -DSIMBODY_HOME=C:\simbody%NUGET_PACKAGE_ID_SUFFIX% -DOPENSIM_DEPENDENCIES_DIR=%OPENSIM_DEPENDENCIES_INSTALL_DIR% -DCMAKE_INSTALL_PREFIX=%OPENSIM_INSTALL_DIR% -DBUILD_JAVA_WRAPPING=ON -DBUILD_PYTHON_WRAPPING=ON -DWITH_BTK:BOOL=ON
 
   # Build.
-  - cmake --build . --config Release -- /maxcpucount:4 /verbosity:quiet #/p:TreatWarningsAsErrors="true"
+  - cmake --build . --config Release -- /maxcpucount:4 /verbosity:quiet
   
 test_script:
   ## Run tests.