MPPL is a robust, command-line preprocessor and portability tool that facilitates the easy porting of code to newer or different platforms. It allows programmers to write Fortran that is more convenient and powerful than Fortran 77 and it produces standard Fortran 77 output. MPPL permits free-form input and structured constructs such as ``while'' and ``for'' loops. It provides nearly all of the new Fortran 90 syntax for real and integer declarations, literal constants, and intrinsic functions, making it easy to write portable code for architectures with different wordsizes. In addition, MPPL has a powerful macro capability, including both built-in and user-defined macros with arguments. The preprocessor and file-inclusion facility encourages structured, easy-to-read programs with fewer labels. MPPL is available on the UNICOS (J90 and SV1) and Unix (AIX, HPUX, IRIX, IRIX64, Linux, OSF1, SunOS 4, and Solaris) systems.