This chapter examines one particular neurological syndrome, semantic refractory access dysphasia, and demonstrates that patients with this disorder can provide a window on the organization of conceptual knowledge. The first attempt to give a principled account of semantic category dissociations was by the contrast of sensory and functional attributes within the domain of animate and inanimate stimuli. The evidence of categorical dissociations derived from refractory access patients must be interpreted with caution. The chapter also examines whether semantic relatedness constitutes a general property of the organization of conceptual space or whether this property is restricted to certain semantic domains. Neuropsychological studies of the nature and organization of conceptual knowledge have tended to concentrate upon certain semantic domains more than for others. Repetitive probe experiments have also led to the proposal of information within conceptual knowledge which is spatially encoded rather than verbally or visually encoded to support geographical knowledge.