Nearly forty years ago — on the publication of Pontryagin’s treatise — control theory was born as a separate mathematical discipline. Much has been accomplished since then, new results have indeed been forthcoming. The theory of necessary conditions has been developed successfully; existence theory is comprehensive and sophisticated; the control of systems described by partial differential equations has been exhaustively studied, and there have been many advances in optimization theory. Indeed, the subject seems to be in a period of consolidation, following an era of change and progress.