Categorization – assimilating objects to psychological equivalence classes – is a crucial cognitive capacity that has always enhanced vertebrate fitness. This chapter reviews from a primate perspective the state of knowledge in comparative categorization’s subdomains: prototypes, exemplars, rules, and abstractions. Primate studies have made a profound contribution to the prototype-exemplar debate – essentially resolving it. They have illuminated the evolutionary emergence of a cognitive capacity for category rules, illuminating also the emergence of humans’ explicit-declarative cognition. In this area, primates appear as a pivotal transitional form. In the literature on abstract concepts (e.g., Same-Different), primate studies highlight the differences in cognitive capacities across vertebrate lines. The review will demonstrate the crucial role of a fitness/ecological perspective in understanding categorization as an adaptive, information-processing capability. It will raise important questions about the similarity structure of natural (and unnatural) kinds and categories. It will show strong continuities between human and animal cognition, but important discontinuities as well. In all the subdomains, the primates have been extraordinary behavioral ambassadors to the broader field of categorization.