In reviewing self-categorization theory and the literature upon which it is based, we conclude that individuals' attempts to form social categories could lead to three kinds of self-categorization. We label them intergroup categorization, ingroup categorization, and outgroup categorization. We review literature supporting these three types and argue that they can help to explain and organize the existing evidence. Moreover, we conclude that distinguishing these three kinds of self-categorization lead to novel predictions regarding social identity, social cognition, and groups. We offer some of those predictions by discussing their potential causes (building from optimal distinctiveness and security seeking literatures) and implications (on topics including prototype complexity, self-stereotyping, stereotype formation, intergroup behavior, dual identity, conformity, and the psychological implications of perceiving uncategorized collections of people). This paper offers a platform from which to build theoretical and empirical advances in social identity, social cognition, and intergroup relations.