This article first focuses on the emergence of a scholarly discourse on periodization. That discourse includes historians' efforts to diversify criteria for individuating periods, and philosophers' analyses of periodization as a form of historiographical theorizing. Next the article turns to the dynamic interaction between scholarly periodization and the broader institutionalization of periodizations. This is followed by a brief review of arguments against periodization. The article ends with a look at how periodizations are treated in knowledge organization systems (KOS).