Idioms are phrases with a figurative meaning that is not evident from the literal meaning of the phrase. Comprehension of idioms in adults varies with compositionality (the extent to which the idiom parts contribute to figurative meaning), 'literality' (the extent to which a literal interpretation is plausible), and the presence of a supportive linguistic context. It is not known how these features affect the development of idiom comprehension. While nondecomposable and decomposable idioms may differ with respect to both acquisition and mature function (e.g., Gibbs et al., 1989a; Gibbs, 1991), the details of acquisition over the school years has not been studied. Literality influences on idiom processing have been described in adults (e.g., Popiel & McRae; Cronk & Schweigert, 1992), but not children. Finally, the underlying neural bases of idiom processing are largely unknown. This thesis reports three studies designed to investigate the normal development, aberrant development, and neural bases of idiom comprehension: a normal developmental study of acquisition of idiom comprehension in typically developing children, a study of aberrant development in children with spina bifida, and a study of developmental brain dysmorphology in the corpus callosum, which is considered important for figurative language. The results of this series of studies provide converging evidence that the two forms of idiom, decomposable and nondecomposable, are developmentally, functionally, and neuroanatomically distinct. Study 1 showed that the two forms of idiom have different acquisition courses. Study 2 showed that the two forms are differently affected by a neurodevelopmental disorder. Study 3 showed that idiom comprehension varied with dysmorphology of the corpus callosum. Further, all three studies suggest that both the literal and figurative meanings of idioms are accessed and must be suppressed during idiom processing in childhood, at least in children with poor comprehension. The results also suggest that the corpus callosum is important for interhemispheric transfer during idiomatic language processing. Moreover, they suggest that both cerebral hemispheres may be important for processing of nondecomposable idioms and more literally plausible idioms, while decomposable idioms and less literally plausible idioms may be processed with less integration of information between the two hemispheres. (Joelene Huber Okrainec)