Behavioral, neuroimaging, and lesion analysis data suggest two parallel semantic systems. One system, with anterior temporal lobe as critical hub, captures taxonomic relations based on feature overlap. A second system, with temporo-parietal junction as critical hub, captures thematic relations based on complementary roles in events. We describe a computational model of this theory that accounted for a one-way behavioral dissociation in aphasic picture naming errors (more taxonomic errors than thematic errors) and a neuroanatomical double dissociation (damaging feature representations led to relatively more taxonomic errors, damaging event representations led to relatively more thematic errors). The model also predicted that both taxonomic and thematic competitors should be automatically activated during single word processing, with taxonomic competitors activated more quickly and more strongly. These predictions were tested and confirmed in a spoken word comprehension experiment using eye tracking to assess the time course of competitor activation.