How is conceptual knowledge organized and represented? Are domains (such as living things) and categories (such as tools, fruit) represented explicitly or can domain and category structure emerge out of a distributed system? Taken at face value, evidence from brain-damaged patients and neuroimaging studies suggests that conceptual knowledge is explicitly structured in independent content-based stores. However, recent analyses of the fine-grained details of semantic impairments, combined with research using connectionist modelling, suggest a different picture - one in which concepts are represented as patterns of activation over multiple semantic properties within a unitary distributed system. Within this context, category-specific deficits emerge as a result of differences in the structure and content of concepts rather than from explicit divisions of conceptual knowledge in separate stores.