I just tried to compile something with the test library and two translation units. Boom! It was this definition, which appears in boost/test/impl/execution_monitor.ipp. signal_handler* signal_handler::s_active_handler = NULL; //!! need to be placed in thread specific storage I can hardly blame Gennadiy, though, having made this mistake myself many times. The reason I noticed this was that I have just started a simple practice that should root out these problems in my own code: I build a program that #includes all my headers in two translation units. For example: // ---- main.cpp ---- #include <boost/python.hpp> extern int f(); int main() { return f(); } // ---- main.cpp ---- #include <boost/python.hpp> int f() { return 0; } Presto! Instant linker errors pointing me at offending definitions. Every library should include a test like the one above. You may be wondering how to work around these issues when you really need header-only capability and can't stick the definition in a source file. In this case, the fix was easy: since the variable in question is a static member, just refactor it into a templated base class. class signal_handler; template <int = 0> struct signal_handler_ { static signal_handler* s_active_handler; }; class signal_handler : signal_handler_<> { ... }; //!! need to be placed in thread specific storage template <int N> signal_handler* signal_handler_<N>::s_active_handler = NULL; When you need a namespace-scope variable, things get a bit more interesting. You might consider sticking one in an unnamed namespace, but then you run a great risk of violating the One Definition Rule (see 3.2 in the standard) when identical template specializations and inline functions that use the variable are instantiated and use distinct objects in two translation units. Boost.Bind's placeholders have this problem. The answer is to initialize a *reference* in the unnamed namespace to a single, common object: namespace whatever { class foo {}; template <class T, int = 0> struct unique_instance { T& get() { static T x; return x; }; }; namespace { foo& bar = unique_instance<foo>::get(); } } -- Dave Abrahams Boost Consulting www.boost-consulting.com