g higher levels of abstraction with automated downward transformations has always been the way to go, as long as the engineers who do the actual work are th the abstractions. A development scheme for complex reactive systems leads from a userfriendly requirements capture method, called play-in scenarios, to full behavioral descriptions of system parts, and from there to final implementation. C O V E R F E A T U R E David Harel The Weizmann Institute of Science An early version of this article appeared in Proc. Fundamental Approaches to Software Eng. (FASE), Lecture Notes in Computer Science, vol. 1783, Springer-Verlag, Berlin, Mar. 2000, pp. 22-34.