Using external, executable subroutines at runtime. An application that depends on dynamic linking calls the external files as needed during execution. The subroutines are typically part of the operating system, but may be auxiliary files that came with the application. .DLL, .SO, .SL or No Extension In the Windows world, these external, executable files are called "dynamic link libraries" (see DLL). In the Unix/Linux world, which includes Mac OS X, the executable files are called "shared libraries" or "shared objects." They are stored in a Library or System folder often without a file extension, although .SO and .SL are also used. The Mac uses .DYLIB extensions for subroutines that are linked at startup rather than as needed. Contrast with static linking.