the preprocessor, as far as "#include" 's go, is a substitution tool. It does paste the code from the included file into the file it is being included in, as stated in a previous post. When you compile code, unless otherwise specified, a obj file is created (for me .obj files are created). The linker then links these .obj files; so that one .obj file may reference code in the other compiled .obj files, in this situation.