In order to cope with the overwhelming diversity of objects and properties in the world, people must mentally group objects, treating them as instances of a category instead of as unique individuals. Many natural categories are hierarchically organized-that is, they are organized into nested class-inclusion relations (e.g., poodle, dog, animal), with some classes being superordinate or subordinate to others. Because hierarchical classification is such a fundamental cognitive process, it has attracted the attention of psychologists interested in cognition and cognitive development. Models of adult concept formation, natural categories, and semantic memory are all concerned to some degree with class inclusion relations. In these models, questions of how information is accessed and organized are of prime concern.