The terms "actor" and "agent" are used widely in the literature on concurrent and distributed programming both because the underlying concepts to which they refer are fundamental, powerful, and interesting and also because these terms admit of varying interpretations in different technical settings. Historically, the terms "actor" and "agent" have a long association. In the definition of an actor, Hewitt wrote that an actor "is a computational agent which has a mail address and a behaviour." [Hewitt 1977]. Some initial study about the relations between actors and agents may be found in [Gasser and Briot 1992]. The ubiquity and malleability of these terms - the term "agent" in particular - are simultaneously a help and a hindrance to those who work in these areas and for us in organizing a series of papers on current research involving actors and agents. It is clearly helpful to avoid introducing new terminology for distinct, but closely related, concepts. However, confusion is fostered when the term has so many meanings that it is difficult to see the common thread which relates them. The intent of the special series of papers on current work with actors and agents is not to resolve the varying meanings of the terms actor and agent, but to illustrate the spectrum of possibilities, to identify some essential similarities among these concepts, and to indicate the relevance of actor and agent research to concurrency.