The literature on the role of models and theories in physics suggests that multiple models, abstract or concrete exist for a particular phenomenon. These models could be organized into hierarchies describing the entire physics domain (Giere 1994). Hierarchies of concepts have been used in other domains with success; however, criteria for a hierarchy of models in physics and theory's role are unclear. Therefore, research was conducted to describe the knowledge structures of experts, intermediates, and novices. A reiterative categorization task required subjects to sort and re-sort physics problems. The proportions of theory- and model-based categories created by the subjects were compared. Novices created hierarchies of model-based categories. Both intermediates and experts created theory-based categories at the highest, most abstract, level but combined model- and theory-based categories at middle and lower levels. These results support a representation of physics knowledge in which a hierarchy of models is organized by theory-based categories.