Categories like ‘noun’ and ‘verb’ represent the basic units of grammar in all human languages, and the retrieval of categorical information associated with words is an essential step in the production of grammatical speech. Studies of brain-damaged patients suggest that knowledge of nouns and verbs can be spared or impaired selectively; however, the functional causes and the neuroanatomical correlates of this dissociation are not well understood: do the dissociations arise because of differences in the way grammatical categories of words are organized in the brain, because of differences in the way grammatical categories are processed in the brain or because of differences in the neural representation of the semantics of nouns and verbs? Although these three possibilities are usually presented as competing hypotheses, I will propose that they may constitute valid explanations for different types of noun-verb dissociations, reflecting the fact that 1) core conceptual properties of nouns and verbs are represented in distinct brain regions and 2) grammatical category is an organizational principle of lexical processing in the brain.