Much of this paper was adapted from the first author's 1992 PhD thesis [Wooldridge, 1992], and as such this work was supported by the UK Science and Engineering Research Council (now the EPSRC). We are grateful to those people that read and commented on earlier drafts of this article, and in particular to the participants of the 1994 workshop on agent theories, architectures, and languages for their encouragement, enthusiasm, and helpful feedback.