This macro (and its friend BOOST_LITTLE_ENDIAN) definition appears in boost/detail/limits.hpp. However, that file is included only if boost/limits.hpp does not detect the standard <limits>. Thus, it does not get defined for more standard compilers. The serialization library makes use of this macro (in a test), so I wonder how it works on standard compilers. I could use the ENDIAN macros in some of my code to provide compile time performance options (linux provides something similar). Is it possible to move the ENDIAN macros into some other header so that they are more easily obtained, and also do not force including a limits workaround? I also imagine if they were actually included in more compilations, that they could get checked for correctness and the maintenance of newer platforms would increase. Thanks!