Purpose: The purpose of this study was to examine children's knowledge of semantic relations. Method: In Experiment 1, the 6-year-olds, 8-year-olds, and adults participated in an object decision task. Participants in the primed group made object decisions in response to primes that were related taxonomically, thematically, or perceptually to the target objects. Those in the unprimed group made decisions about the same stimuli without the benefit of primes. In Experiment 2, the children in the primed group explained the taxonomic and thematic relations between the prime-target pairs used in Experiment 1. Results: In Experiment 1, the strength of semantic relations did not vary with type or age, as taxonomic priming was as strong as thematic priming and the degree of priming did not reliably differentiate the 3 age groups. Differential priming effects between taxonomic and perceptual conditions, the former hastening and the latter slowing responses, suggested that the relation binding object concepts into taxonomies was not reducible to common physical features. In Experiment 2, the 6-year-olds had more difficulty describing taxonomic than thematic relations, whereas the 8-year-olds described both with ease. Conclusions: Contrary to the shift hypothesis, taxonomic and thematic relations structure concepts in children as young as 6 and into adulthood. In accord with the performance hypothesis, 6-year-olds' representations of taxonomic relations are fragile and vulnerable to high task demands.