Abstract: The TAMPR System originated as an approach to the problem of automating the routine modifications of Fortran source programs required to adapt them to a variety of uses or environments [1]**. Overall, the system accomplishes such modifications by applying transformations to Fortran programs at the source level. But the process differs markedly, in detail, from string-based editing or macro expansion. Three steps are involved: (1) A Fortran source program is processed by the TAMPR Recognizer, yielding essentially a parse tree called the abstract form. (2) The Transformation Interpreter applies IGT's (Intragrammatical Transformations) to the abstract form as tree operations [2]. (3) The abstract form is then reconverted to source program form by the Formatter. By ensuring that the transformations are applied only to the correct syntactic entities and only in the intended contexts, the use of the abstract form greatly simplifies establishing the reliability of the overall process.