This volume is an outgrowth of a NATO Advanced Research Workshop on "Expert Judgment and Expert Systems," held in Porto, Portugal, August 1986. Support for the Workshop was provided by the NATO Division of Scientific Affairs, the U.S. Army Research Institute, and the U.S. National Science Foundation. The Workshop brought together researchers from the fields of psychology, decision analysis, and artificial intelligence. The purposes were to assess similarities, differences, and complementarities among the three approaches to the study of expert judgment; to evaluate their relative strengths and weaknesses; and to propose profitable linkages between them. Each of the papers in the present volume is directed toward one or more of those goals. We wish to express our appreciation and thanks to the following persons for their support and assistance: John Adams, Vincent T. Covello, Luis da Cunha, Claire Jeseo, B. Michael Kantrowitz, Margaret Lally, Judith Orasanu, R. M. Rodrigues, and Sandor P. Schuman.