'Mass terms' like water, rice and traffic, have proved very difficult to accommodate in any theory of meaning since, unlike count nouns such as house or dog, they cannot be treated as denoting sets of individuals. In this study, motivated by the need to design a computer program for understanding natural language utterances containing mass terms, Harry Bunt provides a thorough analysis of the problem and offers an original and detailed solution. An extension of classical set theory, Ensemble Theory, is defined. This provides the formal basis of a framework for the analysis of natural language meaning which Dr Bunt calls two-level model-theoretic semantics. The validity of the framework is convincingly demonstrated by the detailed analysis of a fragment of English including sentences with quantified and modified mass terms. This significant advance in our understanding of the formal syntactic and semantic properties of mass terms will be of interest not only to linguists and logicians, but also to all those concerned with the processing of natural language.