diff --git a/src/enable.c b/src/enable.c
index 30e0a086..7a8534a1 100644
--- a/src/enable.c
+++ b/src/enable.c
@@ -184,7 +184,10 @@ static void disableKeyRepeat(_GLFWwindow* window)
 GLFWAPI void glfwEnable(GLFWwindow window, int token)
 {
     if (!_glfwInitialized)
+    {
+        _glfwSetError(GLFW_NOT_INITIALIZED);
         return;
+    }
 
     switch (token)
     {
@@ -216,7 +219,10 @@ GLFWAPI void glfwEnable(GLFWwindow window, int token)
 GLFWAPI void glfwDisable(GLFWwindow window, int token)
 {
     if (!_glfwInitialized)
+    {
+        _glfwSetError(GLFW_NOT_INITIALIZED);
         return;
+    }
 
     switch (token)
     {
diff --git a/src/fullscreen.c b/src/fullscreen.c
index ddeecc53..b2b0117a 100644
--- a/src/fullscreen.c
+++ b/src/fullscreen.c
@@ -44,8 +44,17 @@ GLFWAPI int glfwGetVideoModes(GLFWvidmode* list, int maxcount)
     int count, i, swap, res1, res2, depth1, depth2;
     GLFWvidmode vm;
 
-    if (!_glfwInitialized || maxcount <= 0 || list == NULL)
+    if (!_glfwInitialized)
+    {
+        _glfwSetError(GLFW_NOT_INITIALIZED);
+        return;
+    }
+
+    if (maxcount <= 0 || list == NULL)
+    {
+        // TODO: Figure out if this is an error
         return 0;
+    }
 
     // Get list of video modes
     count = _glfwPlatformGetVideoModes(list, maxcount);
@@ -83,8 +92,17 @@ GLFWAPI int glfwGetVideoModes(GLFWvidmode* list, int maxcount)
 
 GLFWAPI void glfwGetDesktopMode(GLFWvidmode* mode)
 {
-    if (!_glfwInitialized || mode == NULL)
+    if (!_glfwInitialized)
+    {
+        _glfwSetError(GLFW_NOT_INITIALIZED);
         return;
+    }
+
+    if (mode == NULL)
+    {
+        _glfwSetError(GLFW_INVALID_VALUE);
+        return;
+    }
 
     _glfwPlatformGetDesktopMode(mode);
 }
diff --git a/src/glext.c b/src/glext.c
index 65e9c06c..e5078ad5 100644
--- a/src/glext.c
+++ b/src/glext.c
@@ -132,10 +132,19 @@ GLFWAPI int glfwExtensionSupported(const char* extension)
     GLint count;
     int i;
 
-    if (!_glfwInitialized || !_glfwLibrary.window)
+    if (!_glfwInitialized)
+    {
+        _glfwSetError(GLFW_NOT_INITIALIZED);
         return GL_FALSE;
+    }
 
-    _GLFWwindow* window = _glfwLibrary.window;
+    if (!_glfwLibrary.currentWindow)
+    {
+        _glfwSetError(GLFW_NO_CURRENT_WINDOW);
+        return GL_FALSE;
+    }
+
+    _GLFWwindow* window = _glfwLibrary.currentWindow;
 
     // Extension names should not have spaces
     where = (GLubyte*) strchr(extension, ' ');
@@ -184,8 +193,17 @@ GLFWAPI int glfwExtensionSupported(const char* extension)
 
 GLFWAPI void* glfwGetProcAddress(const char* procname)
 {
-    if (!_glfwInitialized || !_glfwLibrary.window)
+    if (!_glfwInitialized)
+    {
+        _glfwSetError(GLFW_NOT_INITIALIZED);
         return NULL;
+    }
+
+    if (!_glfwLibrary.currentWindow)
+    {
+        _glfwSetError(GLFW_NO_CURRENT_WINDOW);
+        return NULL;
+    }
 
     return _glfwPlatformGetProcAddress(procname);
 }
@@ -197,10 +215,19 @@ GLFWAPI void* glfwGetProcAddress(const char* procname)
 
 GLFWAPI void glfwGetGLVersion(int* major, int* minor, int* rev)
 {
-    if (!_glfwInitialized || !_glfwLibrary.window)
+    if (!_glfwInitialized)
+    {
+        _glfwSetError(GLFW_NOT_INITIALIZED);
         return;
+    }
 
-    _GLFWwindow* window = _glfwLibrary.window;
+    if (!_glfwLibrary.currentWindow)
+    {
+        _glfwSetError(GLFW_NO_CURRENT_WINDOW);
+        return;
+    }
+
+    _GLFWwindow* window = _glfwLibrary.currentWindow;
 
     if (major != NULL)
         *major = window->glMajor;
diff --git a/src/input.c b/src/input.c
index 59754162..9164e148 100644
--- a/src/input.c
+++ b/src/input.c
@@ -42,11 +42,18 @@
 GLFWAPI int glfwGetKey(GLFWwindow window, int key)
 {
     if (!_glfwInitialized)
+    {
+        _glfwSetError(GLFW_NOT_INITIALIZED);
         return GLFW_RELEASE;
+    }
 
     // Is it a valid key?
     if (key < 0 || key > GLFW_KEY_LAST)
+    {
+        // TODO: Decide whether key is a value or enum
+        _glfwSetError(GLFW_INVALID_VALUE);
         return GLFW_RELEASE;
+    }
 
     if (window->key[key] == GLFW_STICK)
     {
@@ -66,11 +73,17 @@ GLFWAPI int glfwGetKey(GLFWwindow window, int key)
 GLFWAPI int glfwGetMouseButton(GLFWwindow window, int button)
 {
     if (!_glfwInitialized)
+    {
+        _glfwSetError(GLFW_NOT_INITIALIZED);
         return GLFW_RELEASE;
+    }
 
     // Is it a valid mouse button?
     if (button < 0 || button > GLFW_MOUSE_BUTTON_LAST)
+    {
+        _glfwSetError(GLFW_INVALID_ENUM);
         return GLFW_RELEASE;
+    }
 
     if (window->mouseButton[button] == GLFW_STICK)
     {
@@ -90,7 +103,10 @@ GLFWAPI int glfwGetMouseButton(GLFWwindow window, int button)
 GLFWAPI void glfwGetMousePos(GLFWwindow window, int* xpos, int* ypos)
 {
     if (!_glfwInitialized)
+    {
+        _glfwSetError(GLFW_NOT_INITIALIZED);
         return;
+    }
 
     // Return mouse position
     if (xpos != NULL)
@@ -109,7 +125,10 @@ GLFWAPI void glfwGetMousePos(GLFWwindow window, int* xpos, int* ypos)
 GLFWAPI void glfwSetMousePos(GLFWwindow window, int xpos, int ypos)
 {
     if (!_glfwInitialized)
+    {
+        _glfwSetError(GLFW_NOT_INITIALIZED);
         return;
+    }
 
     // Don't do anything if the mouse position did not change
     if (xpos == window->mousePosX && ypos == window->mousePosY)
@@ -120,7 +139,7 @@ GLFWAPI void glfwSetMousePos(GLFWwindow window, int xpos, int ypos)
     window->mousePosY = ypos;
 
     // If we have a locked mouse, do not change cursor position
-    if (_glfwLibrary.cursorLockWindow)
+    if (_glfwLibrary.cursorLockWindow == window)
         return;
 
     // Update physical cursor position
@@ -135,7 +154,10 @@ GLFWAPI void glfwSetMousePos(GLFWwindow window, int xpos, int ypos)
 GLFWAPI int glfwGetMouseWheel(GLFWwindow window)
 {
     if (!_glfwInitialized)
+    {
+        _glfwSetError(GLFW_NOT_INITIALIZED);
         return 0;
+    }
 
     return window->wheelPos;
 }
@@ -148,7 +170,10 @@ GLFWAPI int glfwGetMouseWheel(GLFWwindow window)
 GLFWAPI void glfwSetMouseWheel(GLFWwindow window, int pos)
 {
     if (!_glfwInitialized)
+    {
+        _glfwSetError(GLFW_NOT_INITIALIZED);
         return;
+    }
 
     window->wheelPos = pos;
 }
@@ -161,7 +186,10 @@ GLFWAPI void glfwSetMouseWheel(GLFWwindow window, int pos)
 GLFWAPI void glfwSetKeyCallback(GLFWwindow window, GLFWkeyfun cbfun)
 {
     if (!_glfwInitialized)
+    {
+        _glfwSetError(GLFW_NOT_INITIALIZED);
         return;
+    }
 
     window->keyCallback = cbfun;
 }
@@ -174,7 +202,10 @@ GLFWAPI void glfwSetKeyCallback(GLFWwindow window, GLFWkeyfun cbfun)
 GLFWAPI void glfwSetCharCallback(GLFWwindow window, GLFWcharfun cbfun)
 {
     if (!_glfwInitialized)
+    {
+        _glfwSetError(GLFW_NOT_INITIALIZED);
         return;
+    }
 
     window->charCallback = cbfun;
 }
@@ -187,7 +218,10 @@ GLFWAPI void glfwSetCharCallback(GLFWwindow window, GLFWcharfun cbfun)
 GLFWAPI void glfwSetMouseButtonCallback(GLFWwindow window, GLFWmousebuttonfun cbfun)
 {
     if (!_glfwInitialized)
+    {
+        _glfwSetError(GLFW_NOT_INITIALIZED);
         return;
+    }
 
     window->mouseButtonCallback = cbfun;
 }
@@ -199,11 +233,14 @@ GLFWAPI void glfwSetMouseButtonCallback(GLFWwindow window, GLFWmousebuttonfun cb
 
 GLFWAPI void glfwSetMousePosCallback(GLFWwindow window, GLFWmouseposfun cbfun)
 {
-    if (!_glfwInitialized || !_glfwLibrary.window)
+    if (!_glfwInitialized)
+    {
+        _glfwSetError(GLFW_NOT_INITIALIZED);
         return;
+    }
 
     // Set callback function
-    _glfwLibrary.window->mousePosCallback = cbfun;
+    window->mousePosCallback = cbfun;
 
     // Call the callback function to let the application know the current
     // mouse position
@@ -219,7 +256,10 @@ GLFWAPI void glfwSetMousePosCallback(GLFWwindow window, GLFWmouseposfun cbfun)
 GLFWAPI void glfwSetMouseWheelCallback(GLFWwindow window, GLFWmousewheelfun cbfun)
 {
     if (!_glfwInitialized)
+    {
+        _glfwSetError(GLFW_NOT_INITIALIZED);
         return;
+    }
 
     // Set callback function
     window->mouseWheelCallback = cbfun;
diff --git a/src/joystick.c b/src/joystick.c
index 7043c3f5..410b4ebb 100644
--- a/src/joystick.c
+++ b/src/joystick.c
@@ -42,7 +42,10 @@
 GLFWAPI int glfwGetJoystickParam(int joy, int param)
 {
     if (!_glfwInitialized)
+    {
+        _glfwSetError(GLFW_NOT_INITIALIZED);
         return 0;
+    }
 
     return _glfwPlatformGetJoystickParam(joy, param);
 }
@@ -57,7 +60,10 @@ GLFWAPI int glfwGetJoystickPos(int joy, float* pos, int numaxes)
     int i;
 
     if (!_glfwInitialized)
+    {
+        _glfwSetError(GLFW_NOT_INITIALIZED);
         return 0;
+    }
 
     // Clear positions
     for (i = 0;  i < numaxes;  i++)
@@ -78,7 +84,10 @@ GLFWAPI int glfwGetJoystickButtons(int joy,
     int i;
 
     if (!_glfwInitialized)
+    {
+        _glfwSetError(GLFW_NOT_INITIALIZED);
         return 0;
+    }
 
     // Clear button states
     for (i = 0;  i < numbuttons;  i++)
diff --git a/src/time.c b/src/time.c
index 957769f9..e47a18ea 100644
--- a/src/time.c
+++ b/src/time.c
@@ -42,7 +42,10 @@
 GLFWAPI double glfwGetTime(void)
 {
     if (!_glfwInitialized)
+    {
+        _glfwSetError(GLFW_NOT_INITIALIZED);
         return 0.0;
+    }
 
     return _glfwPlatformGetTime();
 }
@@ -55,7 +58,10 @@ GLFWAPI double glfwGetTime(void)
 GLFWAPI void glfwSetTime(double time)
 {
     if (!_glfwInitialized)
+    {
+        _glfwSetError(GLFW_NOT_INITIALIZED);
         return;
+    }
 
     _glfwPlatformSetTime(time);
 }
diff --git a/src/window.c b/src/window.c
index 5f85930d..43b90b27 100644
--- a/src/window.c
+++ b/src/window.c
@@ -375,12 +375,25 @@ GLFWAPI GLFWwindow glfwOpenWindow(int width, int height, int mode)
     _GLFWwndconfig wndconfig;
     _GLFWwindow* window;
 
-    if (!_glfwInitialized || _glfwLibrary.window)
+    if (!_glfwInitialized)
+    {
+        _glfwSetError(GLFW_NOT_INITIALIZED);
         return NULL;
+    }
+
+    if (_glfwLibrary.window)
+    {
+        // TODO: Remove this once multi-window is completed
+        _glfwSetError(GLFW_INTERNAL_ERROR);
+        return NULL;
+    }
 
     window = (_GLFWwindow*) malloc(sizeof(_GLFWwindow));
     if (!window)
+    {
+        _glfwSetError(GLFW_OUT_OF_MEMORY);
         return NULL;
+    }
 
     _glfwLibrary.window = window;
 
@@ -418,18 +431,21 @@ GLFWAPI GLFWwindow glfwOpenWindow(int width, int height, int mode)
     {
         // OpenGL 1.x series ended with version 1.5
         glfwCloseWindow(window);
+        _glfwSetError(GLFW_INVALID_VALUE);
         return GL_FALSE;
     }
     else if (wndconfig.glMajor == 2 && wndconfig.glMinor > 1)
     {
         // OpenGL 2.x series ended with version 2.1
         glfwCloseWindow(window);
+        _glfwSetError(GLFW_INVALID_VALUE);
         return GL_FALSE;
     }
     else if (wndconfig.glMajor == 3 && wndconfig.glMinor > 3)
     {
         // OpenGL 3.x series ended with version 3.3
         glfwCloseWindow(window);
+        _glfwSetError(GLFW_INVALID_VALUE);
         return GL_FALSE;
     }
     else
@@ -442,6 +458,7 @@ GLFWAPI GLFWwindow glfwOpenWindow(int width, int height, int mode)
     {
         // Context profiles are only defined for OpenGL version 3.2 and above
         glfwCloseWindow(window);
+        _glfwSetError(GLFW_INVALID_VALUE);
         return GL_FALSE;
     }
 
@@ -449,6 +466,7 @@ GLFWAPI GLFWwindow glfwOpenWindow(int width, int height, int mode)
     {
         // Forward-compatible contexts are only defined for OpenGL version 3.0 and above
         glfwCloseWindow(window);
+        _glfwSetError(GLFW_INVALID_VALUE);
         return GL_FALSE;
     }
 
@@ -456,6 +474,7 @@ GLFWAPI GLFWwindow glfwOpenWindow(int width, int height, int mode)
     {
         // Invalid window mode
         glfwCloseWindow(window);
+        _glfwSetError(GLFW_INVALID_ENUM);
         return GL_FALSE;
     }
 
@@ -506,6 +525,7 @@ GLFWAPI GLFWwindow glfwOpenWindow(int width, int height, int mode)
         // The desired OpenGL version is greater than the actual version
         // This only happens if the machine lacks {GLX|WGL}_ARB_create_context
         glfwCloseWindow(window);
+        _glfwSetError(GLFW_UNAVAILABLE_VERSION);
         return GL_FALSE;
     }
 
@@ -515,6 +535,7 @@ GLFWAPI GLFWwindow glfwOpenWindow(int width, int height, int mode)
         if (!window->GetStringi)
         {
             glfwCloseWindow(window);
+            _glfwSetError(GLFW_INTERNAL_ERROR);
             return GL_FALSE;
         }
     }
@@ -538,7 +559,13 @@ GLFWAPI GLFWwindow glfwOpenWindow(int width, int height, int mode)
 
 GLFWAPI void glfwMakeWindowCurrent(GLFWwindow window)
 {
-    if (!_glfwInitialized || _glfwLibrary.currentWindow == window)
+    if (!_glfwInitialized)
+    {
+        _glfwSetError(GLFW_NOT_INITIALIZED);
+        return;
+    }
+
+    if (_glfwLibrary.currentWindow == window)
         return;
 
     _glfwPlatformMakeWindowCurrent(window);
@@ -553,7 +580,10 @@ GLFWAPI void glfwMakeWindowCurrent(GLFWwindow window)
 GLFWAPI int glfwIsWindow(GLFWwindow window)
 {
     if (!_glfwInitialized)
+    {
+        _glfwSetError(GLFW_NOT_INITIALIZED);
         return GL_FALSE;
+    }
 
     if (window == NULL)
         return GL_FALSE;
@@ -569,7 +599,10 @@ GLFWAPI int glfwIsWindow(GLFWwindow window)
 GLFWAPI void glfwOpenWindowHint(int target, int hint)
 {
     if (!_glfwInitialized)
+    {
+        _glfwSetError(GLFW_NOT_INITIALIZED);
         return;
+    }
 
     switch (target)
     {
@@ -646,7 +679,10 @@ GLFWAPI void glfwOpenWindowHint(int target, int hint)
 GLFWAPI void glfwCloseWindow(GLFWwindow window)
 {
     if (!_glfwInitialized)
+    {
+        _glfwSetError(GLFW_NOT_INITIALIZED);
         return;
+    }
 
     // Show mouse pointer again (if hidden)
     if (window == _glfwLibrary.cursorLockWindow)
@@ -671,7 +707,10 @@ GLFWAPI void glfwCloseWindow(GLFWwindow window)
 GLFWAPI void glfwSetWindowTitle(GLFWwindow window, const char* title)
 {
     if (!_glfwInitialized)
+    {
+        _glfwSetError(GLFW_NOT_INITIALIZED);
         return;
+    }
 
     _glfwPlatformSetWindowTitle(window, title);
 }
@@ -684,7 +723,10 @@ GLFWAPI void glfwSetWindowTitle(GLFWwindow window, const char* title)
 GLFWAPI void glfwGetWindowSize(GLFWwindow window, int* width, int* height)
 {
     if (!_glfwInitialized)
+    {
+        _glfwSetError(GLFW_NOT_INITIALIZED);
         return;
+    }
 
     if (width != NULL)
         *width = window->width;
@@ -700,8 +742,17 @@ GLFWAPI void glfwGetWindowSize(GLFWwindow window, int* width, int* height)
 
 GLFWAPI void glfwSetWindowSize(GLFWwindow window, int width, int height)
 {
-    if (!_glfwInitialized || window->iconified)
+    if (!_glfwInitialized)
+    {
+        _glfwSetError(GLFW_NOT_INITIALIZED);
         return;
+    }
+
+    if (window->iconified)
+    {
+        // TODO: Figure out if this is an error
+        return;
+    }
 
     // Don't do anything if the window size did not change
     if (width == window->width && height == window->height)
@@ -724,8 +775,15 @@ GLFWAPI void glfwSetWindowSize(GLFWwindow window, int width, int height)
 
 GLFWAPI void glfwSetWindowPos(GLFWwindow window, int x, int y)
 {
-    if (!_glfwInitialized || window->mode == GLFW_FULLSCREEN || window->iconified)
+    if (!_glfwInitialized)
     {
+        _glfwSetError(GLFW_NOT_INITIALIZED);
+        return;
+    }
+
+    if (window->mode == GLFW_FULLSCREEN || window->iconified)
+    {
+        // TODO: Figure out if this is an error
         return;
     }
 
@@ -739,7 +797,13 @@ GLFWAPI void glfwSetWindowPos(GLFWwindow window, int x, int y)
 
 GLFWAPI void glfwIconifyWindow(GLFWwindow window)
 {
-    if (!_glfwInitialized || window->iconified)
+    if (!_glfwInitialized)
+    {
+        _glfwSetError(GLFW_NOT_INITIALIZED);
+        return;
+    }
+
+    if (window->iconified)
         return;
 
     _glfwPlatformIconifyWindow(window);
@@ -752,7 +816,13 @@ GLFWAPI void glfwIconifyWindow(GLFWwindow window)
 
 GLFWAPI void glfwRestoreWindow(GLFWwindow window)
 {
-    if (!_glfwInitialized || !window->iconified)
+    if (!_glfwInitialized)
+    {
+        _glfwSetError(GLFW_NOT_INITIALIZED);
+        return;
+    }
+
+    if (!window->iconified)
         return;
 
     // Restore iconified window
@@ -770,7 +840,16 @@ GLFWAPI void glfwRestoreWindow(GLFWwindow window)
 GLFWAPI void glfwSwapBuffers(void)
 {
     if (!_glfwInitialized)
+    {
+        _glfwSetError(GLFW_NOT_INITIALIZED);
         return;
+    }
+
+    if (!_glfwLibrary.currentWindow)
+    {
+        _glfwSetError(GLFW_NO_CURRENT_WINDOW);
+        return;
+    }
 
     if (_glfwLibrary.currentWindow)
         _glfwPlatformSwapBuffers();
@@ -784,7 +863,10 @@ GLFWAPI void glfwSwapBuffers(void)
 GLFWAPI void glfwSwapInterval(int interval)
 {
     if (!_glfwInitialized)
+    {
+        _glfwSetError(GLFW_NOT_INITIALIZED);
         return;
+    }
 
     _glfwPlatformSwapInterval(interval);
 }
@@ -797,7 +879,10 @@ GLFWAPI void glfwSwapInterval(int interval)
 GLFWAPI int glfwGetWindowParam(GLFWwindow window, int param)
 {
     if (!_glfwInitialized)
+    {
+        _glfwSetError(GLFW_NOT_INITIALIZED);
         return 0;
+    }
 
     switch (param)
     {
@@ -860,7 +945,10 @@ GLFWAPI int glfwGetWindowParam(GLFWwindow window, int param)
 GLFWAPI void glfwSetWindowSizeCallback(GLFWwindow window, GLFWwindowsizefun cbfun)
 {
     if (!_glfwInitialized)
+    {
+        _glfwSetError(GLFW_NOT_INITIALIZED);
         return;
+    }
 
     window->windowSizeCallback = cbfun;
 
@@ -877,7 +965,10 @@ GLFWAPI void glfwSetWindowSizeCallback(GLFWwindow window, GLFWwindowsizefun cbfu
 GLFWAPI void glfwSetWindowCloseCallback(GLFWwindow window, GLFWwindowclosefun cbfun)
 {
     if (!_glfwInitialized)
+    {
+        _glfwSetError(GLFW_NOT_INITIALIZED);
         return;
+    }
 
     window->windowCloseCallback = cbfun;
 }
@@ -890,7 +981,10 @@ GLFWAPI void glfwSetWindowCloseCallback(GLFWwindow window, GLFWwindowclosefun cb
 GLFWAPI void glfwSetWindowRefreshCallback(GLFWwindow window, GLFWwindowrefreshfun cbfun)
 {
     if (!_glfwInitialized)
+    {
+        _glfwSetError(GLFW_NOT_INITIALIZED);
         return;
+    }
 
     window->windowRefreshCallback = cbfun;
 }
@@ -903,7 +997,10 @@ GLFWAPI void glfwSetWindowRefreshCallback(GLFWwindow window, GLFWwindowrefreshfu
 GLFWAPI void glfwPollEvents(void)
 {
     if (!_glfwInitialized)
+    {
+        _glfwSetError(GLFW_NOT_INITIALIZED);
         return;
+    }
 
     _glfwPlatformPollEvents();
 }
@@ -916,7 +1013,10 @@ GLFWAPI void glfwPollEvents(void)
 GLFWAPI void glfwWaitEvents(void)
 {
     if (!_glfwInitialized)
+    {
+        _glfwSetError(GLFW_NOT_INITIALIZED);
         return;
+    }
 
     _glfwPlatformWaitEvents();
 }