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Action Semantics is a novel approach to the formal description of programming languages. Its abstractness is at an intermediate level, between that of denotational and operational semantics. Action Semantics has considerable pragmatic advantages over all previous approaches, in its comprehensibility and accessibility, and especially in the usefulness of its semantic descriptions of realistic programming languages. In this volume, Dr Peter Mosses gives a thorough introduction to action semantics, and provides substantial illustrations of its use. Graduates of computer science or maths who have an interest in the semantics of programming languages will find Action Semantics a most helpful book.

Abstract.A novel form of labelled transition system is proposed, where the labels are the arrows of a category, and adjacent labels in computations are required to be composable. Such transition systems provide the foundations for modular SOS descriptions of programming languages. Three fundamental ways of transforming label categories, analogous to monad transformers, are provided, and it is shown that their applications preserve computations in modular SOS. The approach is illustrated with fragments taken from a modular SOS for ML concurrency primitives.