Semantic dementia (SD) is a recently described clinical syndrome characterised by an acquired progressive inability to name or comprehend common concepts, with little or no distortion of the phonological and syntactic aspects of language, and relative sparing of other aspects of cognition, such as episodic memory, nonverbal problem-solving, and perceptual and visuo-spatial skills. The cognitive locus of this syndrome appears to lie in the permanent store of long-term memory representing general world knowledge-semantic memory. The anatomical distribution of atrophy is less well-defined, and the contribution of various imaging modalities is discussed in the context of a body of 45 published and unpublished cases. We conclude that involvement of the left infero-lateral temporal cortex is the critical area in the genesis of SD. SD probably always represents a non-Alzheimer neurodegenerative process; a variety of pathological lesions may be present, and possible causes, together with debates about their correct classification, are discussed.