|There has been a long-standing desire in computer science for a way of collecting and using libraries of standard software components. The limited success in actually doing this stems not from any resistance to the idea, nor from any lack of trying, but rather from the difficulty of choosing an appropriate formalism for representing components. For a formalism to be maximally useful, it must satisfy five key desiderata: expressiveness, convenient combinability, semantic soundness, machine manipulability, and programming language independence. The Plan Calculus formalism developed as part of the Programmer's Apprentice project satisfies each of these desiderata quite well. It does this by combining the ideas from flowchart schemas, data abstraction, logical formalisms, and program transformations. The efficacy of the Plan Calculus has been demonstrated in part by a prototype program editor called the Knowledge- based Editor in Emacs. This editor makes it possible for a programmer to construct a program rapidly and reliably by combining components represented as plans.