The startling discovery by O'Keefe & Dostrovsky (Brain Res. 1971; 34: 171-75) that hippocampal neurons fire selectively in different regions or "place fields" of an environment and the subsequent development of the comprehensive theory by O'Keefe & Nadel (The Hippocampus as a Cognitive Map. Oxford, UK: Clarendon, 1978) that the hippocampus serves as a cognitive map have stimulated a substantial body of literature on the characteristics of hippocampal "place cells" and their relevance for our understanding of the mechanisms by which the brain processes spatial information. This paper reviews the major dimensions of the empirical research on place-cell activity and the development of computational models to explain various characteristics of place fields.