The present crisis in software development forces us to reconsider the fundamental ways in which programming is done. One often quoted solution is to exploit more fully the idea of reusable software. It is the purpose of this paper to examine this concept in all of its forms and to assess the current state of the art. In addition to its usual meaning of reusable code, reusability includes reusable design, various forms of specification systems. so-called application generators, and systems for prototyping. We examine each approach from the perspective of the practicing engineer, and we evaluate the work in terms of how it may ultimately improve the development process for large-scale software systems.