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A substantial literature has examined the nature of social categorization, a fundamental process having important implications for a wide variety of social phenomena. The great majority of this research has focused on the role of particular, clearly identified social categories (e.g. race, nationality, etc.) while ignoring or holding constant other identity dimensions. This approach has afforded considerable leverage for understanding how salient social identities influence perception, judgment, and behavior. However, it leaves unaddressed many questions about how particular social identities become salient and how (and whether) identities might be inferred when category membership is ambiguous or unknown. Everyday social perception often occurs under conditions of volatility (dynamic contexts), uncertainty (missing information), complexity (multiple bases for categorization), and ambiguity (unclear meaning of available cues). As a consequence, research must address how these factors might qualify basic processes of social categorization. Available evidence is reviewed, and directions for future research are discussed. The central importance of social categorization in shaping perception, judgment, and behavior has long been recognized. The writings of seminal theorists such as Allport (1954), Sherif (1948), and Tajfel (1974) emphasized the significance of the psychological borderlines that define membership in particular social groups, and countless studies have examined the consequences of salient category memberships on social functioning (fo