Software components enable practical reuse of software parts and amortization of investments over multiple applications. Each part or component is well defined and independently deployable. Composition is the key technique by which systems of software components are constructed. The composition step can be done before or after the delivery of the system. It is this late composition (or at least the possibility of it) which yields the greatest challenges from a testing standpoint. That is, a component-based application may be composed out of parts that were never tested together. Thus the most useful and reliable parts are those which have been tested independently in as many ways as possible.