This major collection of short essays reviews the scope and progress of research in artificial intelligence over the past two decades. Seminal and most-cited papers from the journal Artificial Intelligence are revisited by the authors who describe how their research has been developed, both by themselves and by others, since the journals first publication.The twenty-eight papers span a wide variety of domains, including truth maintainance systems and qualitative process theory, chemical structure analysis, diagnosis of faulty circuits, and understanding visual scenes; they also span a broad range of methodologies, from AI's mathematical foundations to systems architecture.The volume is dedicated to Allen Newell and concludes with a section of fourteen essays devoted to a retrospective on the strength and vision of his work.Sections/Contributors:- Artificial Intelligence in Perspective, D. G. Bobrow.- Foundations. J. McCarthy, R. C. Moore, A. Newell, N. J. Nilsson, J. Gordon and E. H. Shortliffe, J. Pearl, A. K. Mackworth and E. C. Freuder, J. de Kleer.- Vision. H. G. Barrow and J. M. Tenenbaum, B. K. P. Horn and B. Schunck, K. Ikeuchi, T. Kanade.- Qualitative Reasoning. J. de Kleer, K. D. Forbus, B. J. Kuipers, Y. Iwasake and H. A Simon.- Diagnosis. R. Davis, M. R. Genesereth, P. Szolovits and S. G. Pauker, R. Davis, B. G. Buchanan and E. H. Shortliffe, W. J. Clancey.- Architectures. J. S. Aikins, B. Hayes-Roth, M. J. Stefik et al.- Systems. R. E. Fikes and N. J. Nilsson, E. A Feigenbaum and B. G. Buchanan, J. McDermott. Allen Newell. H. A. Simon, M. J. Stefik and S. W. Smoliar, M. A. Arbib, D. C. Dennett, Purves, R. C. Schank and M. Y. Jona, P. S. Rosenbloom and J. E. Laird, P. E. Agre.