Classic neurological accounts and some contemporary theories of semantic memory assume that concepts are acquired through a learning process that draws together information experienced in each of our verbal and nonverbal modalities. These accounts embody three critical assumptions: semantic representations are amodal; the mapping between surface form and meaning varies for different modalities; and the representations are dynamic. The influence of these three factors was revealed in data collected over a 4-year longitudinal period in two patients with semantic dementia. Semantic assessment revealed a parallel decline in verbal and nonverbal aspects of conceptual knowledge, reflecting a gradual degradation of a single amodal semantic system. As expected, when the patients' semantic impairment was mild, they presented with profound anomia but relatively preserved object use. Over time, performance on all semantic tasks including object use declined. High item-by-item consistency across these tasks was observed in all testing sessions. The impact of dynamic semantic representations was revealed by a striking clinical finding. Although unable to name many of the objects in isolation, their performance was significantly facilitated if they were asked to name while they demonstrated the use of each object. These results are discussed in the context of contemporary models of semantic memory. © 2004 Psychology Press Ltd.