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This paper presents a precise correspondence between two views of taxonomic hierarchies: an intensional view based on concepts and an extensional view based on categories, i.e., subsets of the population of individuals analyzed in terms of these concepts. The correspondence is described with materialization, a generic relationship defined for object-oriented and entity-relationship information models. The paper introduces materialization and shows how it provides a systematic bridge between both views of taxonomies. REAL-WORLD MODELINGWe view the world of interest as consisting of objects (i.e., things, individuals, ideas, ...) important enough to be distinguished from one another. Still, for clarity, we will say that the world is populated by individuals and we will reserve the term object for the usual basic construct of object-oriented models.The world can be described in many ways. We are interested in precise intensional descriptions, called schemas in the database culture, based on concepts that are ideas or notions of various degrees of generality about individuals and sets of individuals in the world. Concepts are used, for example, for distinguishing individuals from other individuals or for characterizing the common properties of similar individuals. Categories are the extensional counterparts of concepts. They serve to classify the population of individuals in the world into subsets perceived as interesting.The activity of conceptual modeling builds such intensional descriptions, also called conceptual models. They are biased and incomplete symbolic images of portions of the world built with concepts. Conceptual models capture meaning in a processable form, in order to perform various tasks of symbolic manipulation regarded as useful (e.g., understand, aggregate, transform information; generalize available information; make assumptions and explore their consequences).Taxonomies are conceptual models. Informally, a taxonomy is an organization of concepts or of categories about individuals in the world structured by an order relation expressing the relative generality of the concepts or categories.