I would like to propose conditional support for #pragma once be added to all header files in boost. This would involve, at the beginning of every header file, a preprocessor check to determine whether or not the particular compiler is on a whitelist of known compilers to optimized #pragma once. If so, #pragma once is used. so, something like this: #if defined(_MSC_VER) /* || defined(OTHER_PRAGMA_ONCE_ENABLED_COMPILER) ... */ #pragma once #endif the rest of the header file, including the original #include guard, could be as normal. to demonstrate the motivation for this, consider the sample on this page: http://www.gamearchitect.net/Articles/ExperimentsWithIncludes.html. At the bottom there is a zip file with a visual studio solution that contains 3 different projects. Each project contains 200 header file, each of which includes all of the other 199 header files. There is a single cpp file with a main function which includes all 200 header files. All 3 projects use internal #include guards. In addition: Project 'RedundantGuard' - Uses external include guards (wrapping the invocation of #include itself inside the appropriate guard) as discussed in the book Large Scale C++ Software Design Project 'PragmaOnce' - Uses #pragma once Project 'Nothing' - Uses nothing additional other than the internal include guards. Here are the build timings on my machine (Visual Studio 2008 Service Pack 2) RedundantGuard - 1 second compile + link time PragmaOnce - 1 second compile + link time Nothing - 15 seconds compile + link time In addition I have tested this in the code base of a commercial product I am working on (~40,000 lines of code). It consists of about 6 different projects, some of which have dependencies between each other and some which don't, so that parallel compilation can happen in some instances RedundantGuard - Not Tested PragmaOnce - 1 minute, 25 seconds compile + link time Nothing - 2 minutes, 1 second compile + link time So, I believe that boost compile times can benefit significantly from this. Thoughts?