Legacy hardware and software systems are defined as those that are currently performing useful tasks, but face possible interruption or termination of operation in the future due to a number of reasons. The "push" reasons include the need for increasing functionality, processing and interface scalability, better form (size, weight, power, volume) requirements, decreased maintenance and lifecycle support costs, and resilience to parts obsolescence. The "pull" reasons can include the availability of superior competing algorithms, architectures, and technologies meeting (or exceeding) the specifications of the legacy system, often at a lower cost. Legacy systems can be found everywhere in the military and commercial electronics area. Indeed, in commercial arena, electronics systems, such as PCs and cellular phones, are often obsolete in a matter of months, and increasing pressures of time-to-market has institutionalized re-engineering of products. In the military arena, the long lifetimes of deployed systems, decades in the case of radar systems, has made it inevitable that one is faced with the problem of legacy systems. It is not an exaggeration to conclude that the question is not "is reengineering necessary ?" but "when and how it should be done".