Before software can be adapted to changing economical, organizational and technical conditions, it has to be ABunderstoodBB. Understanding involves obtaining all the information belonging to the application system in question by means of analysis, interpretation and evaluation of the software's structures (the individual programs of the various program systems plus existing documentation) and of the context and exact nature of its use (e= 2Eg. business processes and workflows). Within a program of research into tool s supporting application understanding, the Fraunhofer ISST has developed a prototype of the FAPU Workbench (FAPU - FORTRAN Application and Program Understanding). A special feature of FAPU is that it distinguishes between program inform ation and non-program information and enables interactive linking within and be tween these two types of information. FAPU can handle files consisting of a mix ture of programs in different languages, control commands and data. Its robust parser can analyse a wide range of FORTRAN dialects and deal with unknown constructs. The location of comments within source code is preserved, new comments can be added and existing ones modified. The analysis is always performed with respect to a platform model containing information about t he computer type, the operating system and the compiler. As well as many analysis options and the synchronization of their presentation, FAPU also enables visualization of COMMON blocks. This paper presents the tool developed at the Fraunhofer ISST.