ctly with the computer’s processor with very primitive instructions. Since all that a computer can really understand are these primitive machine language instructions, a Fortran program must be translated into machine language by a special program called a Fortran compiler before it can be executed. Since the processors in various computers are not all the same, their machine languages are not all the same. For a variety of reasons, not all Fortran compilers are the same. For example, more recent Fortran compilers allow operations not allowed by earlier versions. In this chapter, we will only describe features that one can expect to have available with whatever compiler one may have available. Fortran was initially developed almost exclusively for performing numeric computations (Fortran is an acronym for “Formula Translation”), and a host of other languages (Pascal, Ada, Cobol, C, etc.) have been developed that are more suited to nonnumerical operations such as searching databases for information. Fortran has managed to adapt itself to the changing nature of computing and has survived, despite repeated predictions of its death. It is still the major language of science and is heavily used in statistical computing. The most standard version of Fortran is referred to as Fortran 77 since it is based on a standard established in 1977. A new standard was developed in 1990 that incorporates some of the useful ideas from other languages but we will restrict ourselves to Fortran 77