Background: Figurative language, such as metaphor and metonymy, is very common in daily language use. Its underlying cognitive processes are sometimes viewed as lying at the interface of language and thought. Williams syndrome, which is a rare genetic developmental disorder, provides an opportunity to study this interface because individuals with this disorder have relative strengths in vocabulary and syntax against a background of low general cognitive ability. Few studies have investigated metaphor comprehension in Williams syndrome and none has investigated metonymy. Aims: This is the first study to investigate metaphor and metonymy comprehension in Williams syndrome and to compare their performance with a group of typically developing children. Methods & Procedures: Ten children with Williams syndrome were compared with eleven typically developing children in a novel metaphor-metonymy comprehension task. Cross-sectional trajectory analyses were used to compare the development of metaphor and metonymy using a child-friendly story picture task. Trajectories were constructed linking task performance either to chronological age or to measures of mental age (receptive vocabulary, visuospatial construction). Outcomes & Results: The performance of children with Williams syndrome was significantly poorer than the typically developing group. The comprehension of metonyms was in line with receptive vocabulary, but comprehension of metaphors fell below this level. Conclusions & Implications: Metonyms may be part of vocabulary and treated as synonyms in Williams syndrome, while metaphor engages additional cognitive mechanisms outside language that develop atypically in this disorder. Despite earlier reports that emphasize good language skills, the Williams syndrome language system shows anomalies compared with typical development. (Contains 1 table, 4 figures, and 1 note.)