diff --git a/.circleci/config.yml b/.circleci/config.yml
index 89ff50941..79f318f20 100644
--- a/.circleci/config.yml
+++ b/.circleci/config.yml
@@ -26,14 +26,16 @@ jobs:
           command: |
             mkdir -p release
             cd release
-            env CFLAGS="-Wdeclaration-after-statement -Werror" CXXFLAGS="-Werror" cmake ..
+            env CFLAGS="-Wdeclaration-after-statement -Werror" CXXFLAGS="-Werror" cmake -Werror=dev -DDOWNLOAD_GTEST=ON ..
             make everything
+            make run_tests
             ./teeworlds_srv shutdown
       - run:
           name: Build teeworlds with cmake in Debug mode
           command: |
             mkdir -p debug
             cd debug
-            env CFLAGS="-Wdeclaration-after-statement -Werror" CXXFLAGS="-Werror" cmake -DDEV=ON ..
+            env CFLAGS="-Wdeclaration-after-statement -Werror" CXXFLAGS="-Werror" cmake -Werror=dev -DDOWNLOAD_GTEST=ON -DDEV=ON ..
             make everything
+            make run_tests
             ./teeworlds_srv shutdown
diff --git a/CMakeLists.txt b/CMakeLists.txt
index 547d9851b..20e46dd55 100644
--- a/CMakeLists.txt
+++ b/CMakeLists.txt
@@ -66,6 +66,7 @@ endif()
 
 option(CLIENT "Compile client" ON)
 option(DOWNLOAD_DEPENDENCIES "Download dependencies (only available on Windows)" ${AUTO_DEPENDENCIES_DEFAULT})
+option(DOWNLOAD_GTEST "Download and compile GTest if not found" ${AUTO_DEPENDENCIES_DEFAULT})
 option(PREFER_BUNDLED_LIBS "Prefer bundled libraries over system libraries" ${AUTO_DEPENDENCIES_DEFAULT})
 option(DEV "Don't generate stuff necessary for packaging" OFF)
 
@@ -285,6 +286,7 @@ endif()
 find_package(ZLIB)
 find_package(Freetype)
 find_package(Git)
+find_package(GTest)
 find_package(Pnglite)
 find_package(SDL2)
 find_package(Threads)
@@ -346,6 +348,18 @@ endif()
 if(CLIENT AND NOT(SDL2_FOUND))
   message(SEND_ERROR "You must install SDL2 to compile the Teeworlds client")
 endif()
+if(NOT(GTEST_FOUND))
+  if(DOWNLOAD_GTEST)
+    if(GIT_FOUND)
+      message(STATUS "Automatically downloading GTest to be able to run tests")
+    else()
+      set(DOWNLOAD_GTEST OFF)
+      message(WARNING "To automatically download GTest, you have to install Git")
+    endif()
+  else()
+    message(STATUS "To run the tests, you have to install GTest")
+  endif()
+endif()
 
 if(TARGET_OS STREQUAL "windows")
   set(PLATFORM_CLIENT)
@@ -371,6 +385,57 @@ else()
   endif()
 endif()
 
+########################################################################
+# DOWNLOAD GTEST
+########################################################################
+
+if(NOT(GTEST_FOUND) AND DOWNLOAD_GTEST)
+  set(TEEWORLDS_GTEST_VERSION release-1.8.1)
+  configure_file(cmake/Download_GTest_CMakeLists.txt.in googletest-download/CMakeLists.txt)
+  execute_process(COMMAND ${CMAKE_COMMAND} -G "${CMAKE_GENERATOR}" .
+    RESULT_VARIABLE result
+    WORKING_DIRECTORY ${PROJECT_BINARY_DIR}/googletest-download
+  )
+  if(result)
+    message(WARNING "CMake step for googletest failed: ${result}")
+    set(DOWNLOAD_GTEST OFF)
+  else()
+    execute_process(COMMAND ${CMAKE_COMMAND} --build .
+      RESULT_VARIABLE result
+      WORKING_DIRECTORY ${PROJECT_BINARY_DIR}/googletest-download
+    )
+    if(result)
+      message(WARNING "Build step for googletest failed: ${result}")
+      set(DOWNLOAD_GTEST OFF)
+    else()
+      # Prevent overriding the parent project's compiler/linker settings on Windows
+      set(gtest_force_shared_crt ON CACHE BOOL "" FORCE)
+
+      # Add googletest directly to our build. This defines the gtest target.
+      add_subdirectory(
+        ${PROJECT_BINARY_DIR}/googletest-src
+        ${PROJECT_BINARY_DIR}/googletest-build
+        EXCLUDE_FROM_ALL
+      )
+
+      if(MSVC)
+        foreach(target gtest)
+          # `/w` disables all warnings. This is needed because `gtest` enables
+          # `/WX` (equivalent of `-Werror`) for some reason, breaking builds
+          # when MSVS adds new warnings.
+          target_compile_options(${target} PRIVATE $<$<NOT:${DBG}>:/MT> $<${DBG}:/MTd> /w)
+        endforeach()
+      endif()
+
+      set(GTEST_LIBRARIES gtest)
+      set(GTEST_INCLUDE_DIRS)
+      if(CMAKE_VERSION VERSION_LESS 2.8.11)
+        set(GTEST_INCLUDE_DIRS "${gtest_SOURCE_DIR}/include")
+      endif()
+    endif()
+  endif()
+endif()
+
 ########################################################################
 # DEPENDENCY COMPILATION
 ########################################################################
@@ -1478,6 +1543,40 @@ list(APPEND TARGETS_LINK ${TARGETS_TOOLS})
 add_custom_target(tools DEPENDS ${TARGETS_TOOLS})
 add_custom_target(everything DEPENDS ${TARGETS_OWN})
 
+########################################################################
+# TESTS
+########################################################################
+
+if(GTEST_FOUND OR DOWNLOAD_GTEST)
+  set_src(TESTS GLOB src/test
+    fs.cpp
+    git_revision.cpp
+    str.cpp
+    test.cpp
+    test.h
+    thread.cpp
+  )
+  set(TARGET_TESTRUNNER testrunner)
+  add_executable(${TARGET_TESTRUNNER} EXCLUDE_FROM_ALL
+    ${TESTS}
+    $<TARGET_OBJECTS:engine-shared>
+    $<TARGET_OBJECTS:game-shared>
+    ${DEPS}
+  )
+  target_link_libraries(${TARGET_TESTRUNNER} ${LIBS} ${GTEST_LIBRARIES})
+  target_include_directories(${TARGET_TESTRUNNER} PRIVATE ${GTEST_INCLUDE_DIRS})
+
+  list(APPEND TARGETS_OWN ${TARGET_TESTRUNNER})
+  list(APPEND TARGETS_LINK ${TARGET_TESTRUNNER})
+
+  add_custom_target(run_tests
+    COMMAND $<TARGET_FILE:${TARGET_TESTRUNNER}> ${TESTRUNNER_ARGS}
+    COMMENT Running tests
+    DEPENDS ${TARGET_TESTRUNER}
+    USES_TERMINAL
+  )
+endif()
+
 ########################################################################
 # INSTALLATION
 ########################################################################
diff --git a/appveyor.yml b/appveyor.yml
index 2b5e5b737..d864cf9d9 100644
--- a/appveyor.yml
+++ b/appveyor.yml
@@ -17,6 +17,10 @@ build_script:
   - cmd: cmake --build build64 --config Release --target everything
 
 test_script:
+  - cmd: cmake --build build32 --config Debug --target run_tests
+  - cmd: cmake --build build64 --config Debug --target run_tests
+  - cmd: cmake --build build32 --config Release --target run_tests
+  - cmd: cmake --build build64 --config Release --target run_tests
   - cmd: |
       cd build32
       Release\teeworlds_srv shutdown
diff --git a/cmake/Download_GTest_CMakeLists.txt.in b/cmake/Download_GTest_CMakeLists.txt.in
new file mode 100644
index 000000000..c1a176b02
--- /dev/null
+++ b/cmake/Download_GTest_CMakeLists.txt.in
@@ -0,0 +1,16 @@
+cmake_minimum_required(VERSION 2.8)
+
+project(googletest-download NONE)
+
+include(ExternalProject)
+ExternalProject_Add(googletest
+  GIT_REPOSITORY    https://github.com/google/googletest.git
+  GIT_TAG           "${TEEWORLDS_GTEST_VERSION}"
+  SOURCE_DIR        "${CMAKE_BINARY_DIR}/googletest-src"
+  BINARY_DIR        "${CMAKE_BINARY_DIR}/googletest-build"
+  CONFIGURE_COMMAND ""
+  BUILD_COMMAND     ""
+  INSTALL_COMMAND   ""
+  TEST_COMMAND      ""
+  TLS_VERIFY        ON
+)
diff --git a/src/base/system.c b/src/base/system.c
index 2f222a0a5..f200055a7 100644
--- a/src/base/system.c
+++ b/src/base/system.c
@@ -39,6 +39,7 @@
 	#include <fcntl.h>
 	#include <direct.h>
 	#include <errno.h>
+	#include <process.h>
 	#include <wincrypt.h>
 #else
 	#error NOT IMPLEMENTED
@@ -99,7 +100,7 @@ void dbg_msg(const char *sys, const char *fmt, ...)
 
 	va_start(args, fmt);
 #if defined(CONF_FAMILY_WINDOWS)
-	_vsnprintf(msg, sizeof(str)-len, fmt, args);
+	_vsprintf_p(msg, sizeof(str)-len, fmt, args);
 #else
 	vsnprintf(msg, sizeof(str)-len, fmt, args);
 #endif
@@ -421,7 +422,7 @@ unsigned io_write_newline(IOHANDLE io)
 int io_close(IOHANDLE io)
 {
 	fclose((FILE*)io);
-	return 1;
+	return 0;
 }
 
 int io_flush(IOHANDLE io)
@@ -1471,7 +1472,7 @@ int fs_storage_path(const char *appname, char *path, int max)
 	char *home = getenv("APPDATA");
 	if(!home)
 		return -1;
-	_snprintf(path, max, "%s/%s", home, appname);
+	str_format(path, max, "%s/%s", home, appname);
 	return 0;
 #else
 	char *home = getenv("HOME");
@@ -1483,25 +1484,25 @@ int fs_storage_path(const char *appname, char *path, int max)
 		return -1;
 	
 #if defined(CONF_PLATFORM_MACOSX)
-	snprintf(path, max, "%s/Library/Application Support/%s", home, appname);
+	str_format(path, max, "%s/Library/Application Support/%s", home, appname);
 	return 0;
 #endif
 
 	/* old folder location */
-	snprintf(path, max, "%s/.%s", home, appname);
+	str_format(path, max, "%s/.%s", home, appname);
 	for(i = strlen(home)+2; path[i]; i++)
 		path[i] = tolower(path[i]);
 
 	if(!xdgdatahome)
 	{
 		/* use default location */
-		snprintf(xdgpath, max, "%s/.local/share/%s", home, appname);
+		str_format(xdgpath, max, "%s/.local/share/%s", home, appname);
 		for(i = strlen(home)+14; xdgpath[i]; i++)
 			xdgpath[i] = tolower(xdgpath[i]);
 	}
 	else
 	{
-		snprintf(xdgpath, max, "%s/%s", xdgdatahome, appname);
+		str_format(xdgpath, max, "%s/%s", xdgdatahome, appname);
 		for(i = strlen(xdgdatahome)+1; xdgpath[i]; i++)
 			xdgpath[i] = tolower(xdgpath[i]);
 	}
@@ -1514,7 +1515,7 @@ int fs_storage_path(const char *appname, char *path, int max)
 		return 0;
 	}
 	
-	snprintf(path, max, "%s", xdgpath);
+	str_format(path, max, "%s", xdgpath);
 
 	return 0;
 #endif
@@ -1773,7 +1774,7 @@ void str_format(char *buffer, int buffer_size, const char *format, ...)
 #if defined(CONF_FAMILY_WINDOWS)
 	va_list ap;
 	va_start(ap, format);
-	_vsnprintf(buffer, buffer_size, format, ap);
+	_vsprintf_p(buffer, buffer_size, format, ap);
 	va_end(ap);
 #else
 	va_list ap;
@@ -2351,6 +2352,15 @@ void secure_random_fill(void *bytes, unsigned length)
 #endif
 }
 
+int pid()
+{
+#if defined(CONF_FAMILY_WINDOWS)
+	return _getpid();
+#else
+	return getpid();
+#endif
+}
+
 #if defined(__cplusplus)
 }
 #endif
diff --git a/src/base/system.h b/src/base/system.h
index bd46dc2e1..a21e4b811 100644
--- a/src/base/system.h
+++ b/src/base/system.h
@@ -1441,6 +1441,15 @@ int secure_random_init();
 */
 void secure_random_fill(void *bytes, unsigned length);
 
+/*
+	Function: pid
+		Gets the process ID of the current process
+
+	Returns:
+		The process ID of the current process.
+*/
+int pid();
+
 #ifdef __cplusplus
 }
 #endif
diff --git a/src/test/fs.cpp b/src/test/fs.cpp
new file mode 100644
index 000000000..fc10c7ed4
--- /dev/null
+++ b/src/test/fs.cpp
@@ -0,0 +1,14 @@
+#include "test.h"
+#include <gtest/gtest.h>
+
+#include <base/system.h>
+
+TEST(Filesystem, CreateCloseDelete)
+{
+	CTestInfo Info;
+
+	IOHANDLE File = io_open(Info.m_aFilename, IOFLAG_WRITE);
+	ASSERT_TRUE(File);
+	EXPECT_FALSE(io_close(File));
+	EXPECT_FALSE(fs_remove(Info.m_aFilename));
+}
diff --git a/src/test/git_revision.cpp b/src/test/git_revision.cpp
new file mode 100644
index 000000000..3389becd0
--- /dev/null
+++ b/src/test/git_revision.cpp
@@ -0,0 +1,14 @@
+#include <gtest/gtest.h>
+
+#include <base/system.h>
+#include <game/version.h>
+
+extern const char *GIT_SHORTREV_HASH;
+
+TEST(GitRevision, ExistsOrNull)
+{
+	if(GIT_SHORTREV_HASH)
+	{
+		ASSERT_STRNE(GIT_SHORTREV_HASH, "");
+	}
+}
diff --git a/src/test/str.cpp b/src/test/str.cpp
new file mode 100644
index 000000000..0e3884646
--- /dev/null
+++ b/src/test/str.cpp
@@ -0,0 +1,63 @@
+#include <gtest/gtest.h>
+
+#include <base/system.h>
+
+TEST(Str, Startswith)
+{
+	EXPECT_TRUE(str_startswith("abcdef", "abc"));
+	EXPECT_FALSE(str_startswith("abc", "abcdef"));
+
+	EXPECT_TRUE(str_startswith("xyz", ""));
+	EXPECT_FALSE(str_startswith("", "xyz"));
+
+	EXPECT_FALSE(str_startswith("house", "home"));
+	EXPECT_FALSE(str_startswith("blackboard", "board"));
+
+	EXPECT_TRUE(str_startswith("поплавать", "по"));
+	EXPECT_FALSE(str_startswith("плавать", "по"));
+
+	static const char ABCDEFG[] = "abcdefg";
+	static const char ABC[] = "abc";
+	EXPECT_EQ(str_startswith(ABCDEFG, ABC) - ABCDEFG, str_length(ABC));
+}
+
+TEST(Str, Endswith)
+{
+	EXPECT_TRUE(str_endswith("abcdef", "def"));
+	EXPECT_FALSE(str_endswith("def", "abcdef"));
+
+	EXPECT_TRUE(str_endswith("xyz", ""));
+	EXPECT_FALSE(str_endswith("", "xyz"));
+
+	EXPECT_FALSE(str_endswith("rhyme", "mine"));
+	EXPECT_FALSE(str_endswith("blackboard", "black"));
+
+	EXPECT_TRUE(str_endswith("люди", "юди"));
+	EXPECT_FALSE(str_endswith("люди", "любовь"));
+
+	static const char ABCDEFG[] = "abcdefg";
+	static const char DEFG[] = "defg";
+	EXPECT_EQ(str_endswith(ABCDEFG, DEFG) - ABCDEFG,
+		str_length(ABCDEFG) - str_length(DEFG));
+}
+
+TEST(StrFormat, Positional)
+{
+	char aBuf[256];
+
+	// normal
+	str_format(aBuf, sizeof(aBuf), "%s %s", "first", "second");
+	EXPECT_STREQ(aBuf, "first second");
+
+	// normal with positional arguments
+	str_format(aBuf, sizeof(aBuf), "%1$s %2$s", "first", "second");
+	EXPECT_STREQ(aBuf, "first second");
+
+	// reverse
+	str_format(aBuf, sizeof(aBuf), "%2$s %1$s", "first", "second");
+	EXPECT_STREQ(aBuf, "second first");
+
+	// duplicate
+	str_format(aBuf, sizeof(aBuf), "%1$s %1$s %2$d %1$s %2$d", "str", 1);
+	EXPECT_STREQ(aBuf, "str str 1 str 1");
+}
diff --git a/src/test/test.cpp b/src/test/test.cpp
new file mode 100644
index 000000000..761d661f7
--- /dev/null
+++ b/src/test/test.cpp
@@ -0,0 +1,19 @@
+#include "test.h"
+#include <gtest/gtest.h>
+
+#include <base/system.h>
+
+CTestInfo::CTestInfo()
+{
+	const ::testing::TestInfo *pTestInfo =
+		::testing::UnitTest::GetInstance()->current_test_info();
+	str_format(m_aFilename, sizeof(m_aFilename), "%s.%s-%d.tmp",
+		pTestInfo->test_case_name(), pTestInfo->name(), pid());
+}
+
+int main(int argc, char **argv)
+{
+	::testing::InitGoogleTest(&argc, argv);
+	net_init();
+	return RUN_ALL_TESTS();
+}
diff --git a/src/test/test.h b/src/test/test.h
new file mode 100644
index 000000000..71f13c9b0
--- /dev/null
+++ b/src/test/test.h
@@ -0,0 +1,9 @@
+#ifndef TEST_TEST_H
+#define TEST_TEST_H
+class CTestInfo
+{
+public:
+	CTestInfo();
+	char m_aFilename[64];
+};
+#endif // TEST_TEST_H
diff --git a/src/test/thread.cpp b/src/test/thread.cpp
new file mode 100644
index 000000000..4f2946d4a
--- /dev/null
+++ b/src/test/thread.cpp
@@ -0,0 +1,48 @@
+#include <gtest/gtest.h>
+
+#include <base/system.h>
+
+static void Nothing(void *pUser)
+{
+	(void)pUser;
+}
+
+TEST(Thread, Detach)
+{
+	void *pThread = thread_init(Nothing, 0);
+	thread_detach(pThread);
+}
+
+static void SetToOne(void *pUser)
+{
+	*(int *)pUser = 1;
+}
+
+TEST(Thread, Wait)
+{
+	int Integer = 0;
+	void *pThread = thread_init(SetToOne, &Integer);
+	thread_wait(pThread);
+	EXPECT_EQ(Integer, 1);
+}
+
+TEST(Thread, Yield)
+{
+	thread_yield();
+}
+
+static void LockThread(void *pUser)
+{
+	LOCK *pLock = (LOCK *)pUser;
+	lock_wait(*pLock);
+	lock_unlock(*pLock);
+}
+
+TEST(Thread, Lock)
+{
+	LOCK Lock = lock_create();
+	lock_wait(Lock);
+	void *pThread = thread_init(LockThread, &Lock);
+	lock_unlock(Lock);
+	thread_wait(pThread);
+}