This thesis explores the development of lexicalised as well as novel metaphor and metonymy comprehension in typically developing (TD) children and children with Williams Syndrome (WS). Generally, understanding of figurative language requires understanding of intentions, wide encyclopaedic knowledge, and semantic categories. While Social Inference Theory claims impairments can be due to the ability to infer mental states, Weak Central Coherence Theory (WCC) suggests impairments are caused by inability to use context to derive meaning. However, previous studies have not used a comprehensive battery of tests to try to identify the cognitive abilities related to comprehension of metaphors and metonyms. The current thesis is innovative as 1) it seeks to construct developmental trajectories in order to study the development of metaphor and metonymy comprehension, 2) it compares the comprehension of novel to lexicalised expressions in that focusing on lexicalised expressions might not be fully informative about when and how figurative expressions are understood when first encountered, and 3) it relates performance on metaphor and metonymy understanding to multiple possible underlying factors in order to investigate the cognitive abilities that drive figurative language development and comprehension in the TD and WS groups. -- Two decision tasks, one with lexicalised and one with novel expressions, were constructed in which participants listened to short stories containing a figurative expression. The results indicate that performance on lexicalised as well as novel metaphors and metonyms increased with increasing chronological age in TD children. WS participants showed a delayed development of lexicalised expressions while performance on novel expressions was delayed and atypical.