Although the concept of niche has been extremely useful in sociological theory and research, some aspects of the concept have not been clearly developed. This article advances a theoretical reconstruction of the concept of niche, with special application to organizations. The proposed formal model unifies several active lines of sociological theory. It also extends the notion of the niche from the realm of behaviors to apply to the rules coding social identities and organizational forms. The reconstruction gives deeper insight into the niche of an organizational population as well as individual organizations. Finally, the model analyzes the (thus far) tacit assumption that niches are convex, examines the implications of convexity for commonly used measures of niche width, and provides a general sociological argument for the predominance of convex niches.