The transition from sequential to parallel computation is an area of critical concern in today's computer technology, particularly in architecture, programming languages, systems, and artificial intelligence. This book addresses central issues in concurrency, and by producing both a syntactic definition and a denotational model of Hewitt's actor paradigm -- a model of computation specifically aimed at constructing and analyzing distributed large-scale parallel systems -- it substantially advances the understanding of parallel computation. Contents: Introduction. General Design Decisions. Computation in ACTOR Systems. A More Expressive Language. A Model for ACTOR Systems. Concurrency Issues. Abstraction and Compositionality. Conclusions.