It is easy to start with the truism that all things can be categorized in multiple ways. Unlike some other truisms that can be made about the remarkable and ubiquitous process of categorization, such an opening statement risks serious understatement. Beyond the simple observation that people are capable of using many different categorizations of the same thing, we can recognize that unless people were able to categorize any given object not just in many different ways, but in an infinite number of different ways, this process would not be anywhere near as useful as it is. Indeed, we can interpret the opening statement as being anything from a bland truism to a grand (and potentially controversial) statement about categorization. The power of categorization emerges primarily because things can be categorized in many different ways. That is, at two different times an object can be assigned to different categories (implying we need to understand category selection or application), and at any given time, that object may be simultaneously assigned to more than one category, all of which may affect the way we perceive that object (implying we need to understand how categories are combined or category conjunction; see chapters by Crisp & Hewstone; Smith, this volume). Both of these points suggest that beyond understanding individual categories we need to understand systems of categories. This chapter is about ways of understanding the structure and nature of social categorical systems.