Categorization is the process of understanding things by knowing what other things they are equivalent to and different from. It is a process that is widely studied in cognitive and social psychology and in philosophy and linguistics. This entry offers an overview of categorization and outlines its functions, then describes the major views on category structure and representation and on category learning, formation, and use (drawn mainly from cognitive psychology). It also examines social psychological perspectives on biased stimulus processing and sense making, and concludes by considering a range of ongoing debates and controversies in the field.