Legacy systems abound in the Navy. In many cases it is infeasible or ill advised to develop new systems from scratch. Yet due to new system requirements, increasing maintenance costs, or other reasons it is desirable to somehow replace or upgrade these systems. There is also a desire to capture some of the capabilities inherent in these systems and somehow reuse them. This paper looks at some of the issues involved reengineering, and in particular those that tend to be unique to or more pronounced in Navy systems. Most legacy Navy tactical systems run on Navy military standard computers such as the UYK-43, UYK-44, and AYK-14 using code written the Navy standard CMS-2 programming language. In order to make use of current and future technology in both hardware and software it is desirable to more off of these platforms to commercial processor families, and move from CMS-2 to a more widely supported, contemporary language. At present the language of choice would appear to be Ada, but almost all of the following discussions apply equally well to C or any other equivalent language.This paper is concerned primarily with real time, safety critical, mission critical, complex systems. Most of these types of systems involve the use of computers. These programs run on Navy military standard computers and use Navy military standard displays and peripherals.Many of these systems were designed with memory or other architectural constraints which no longer apply. Thus thorough reengineering of them requires consideration of the original design rationale which may not be explicit in the existing documentation. Moreover, there may be timing or other relationships which only become apparent during the most thorough system test.