Over the past two decades, Ray Jackendoff has persistently tackled difficult issues in the theory of mind and related theories of cognitive processing. Chief among his contributions is a formal theory that elaborates the nature of language and its relationship to a broad set of other domains. "Languages of the Mind" provides convenient access to Jackendoff's work over the past five years on the nature of mental representations in a variety of cognitive domains, in the context of a detailed theory of the level of conceptual structure developed in his earlier books, "Semantics and Cognition" and "Consciousness and the Computational Mind". The first two chapters summarize the theory of levels of mental representation ("languages of the mind") and their relationship to each other and show how conceptual structure can be approached along lines familiar from syntactic and phonological theory. From this background, subsequent chapters develop issues in word learning (and its pertinence to the Piaget-Chomsky debate) and the relation of conceptual structure to the understanding of physical space.