This thesis is concerned with the structure of natural categories. The traditional approach was that category exemplars are equal in terms of membership and that definite boundaries exist between category members and non-members. A view now widely held is that categories are highly structured with well-defined cores, usually consisting of prototypical instances, and fuzzy boundaries. One question which has arisen is whether children also have categories which are structured in this way. This question has been addressed here by conducting a series of category sorting experiments. The results of the first, second and third experiments reported showed that children from three years of age do indeed have categories with well-defined cores and fuzzy boundaries, and that children and adults are influenced by exemplar typicality. The fourth experiment showed that children and adults use the same criteria for categorising when competing criteria are presented. The fifth, sixth, seventh and eighth experiments investigated the role played by locational information and order of presentation on judgements of category membership. Both location and order were shown to influence decisions about category membership. The younger children in particular were affected by order and locational information.