Originally published in 1978, the papers in this book derive from a 1976 meeting sponsored by the Social Science Research Council to discuss the nature and principles of category formation. It is organized in three sections: real-world categories, the cognitive processes underlying categorization, and the nature of representation. Part I examines different structural aspects of real-world categories: folk biological taxonomies, within and between category structures for material objects, and some categories in a language that codes the world in a visual-gestural mode. All three chapters in Part I assume category processors who are able to perform at least three cognitive functions: They can judge similarity between stimuli; they can perceive and process the attributes of a stimulus; and they can learn. Part II presents analyses of these three cognitive functions. All discussion of psychological structures and processes lead eventually to the issue of representation, and Part III examines representational assumptions underlying the earlier discussions. Today it can be read and enjoyed in its historical context.