Hi, Sean Farrow. I use Boost.Asio with Qt. I don't use the default MSVS C++ project structure with additional header, where MSVS adds some preprocessor directives by default. So I moved those directives (preprocessor defines) to project settings: WIN32 WIN32_LEAN_AND_MEAN WINVER=0x0500 _WIN32_WINNT=0x0500 _WIN32_WINDOWS=0x0410 _WIN32_IE=0x0600 May be would solve your problem? Regards, Marat Abrarov.