Fragile X syndrome is the leading inherited cause of intellectual disability. Most boys with fragile X syndrome have impaired cognition and language deficits, with significant within-syndrome variability. Syntax may be especially delayed relative to nonverbal cognition; however, little is known about the specificity of delay, the sources of that difficulty, or the impact of those delays on other aspects of language development. Comparisons among boys with fragile X syndrome, idiopathic autism spectrum disorder, and typical development were made to assess syntactic comprehension, the extension of syntactic knowledge to novel verbs, and distinct cognitive abilities that might predict language ability and learning in three studies. In Study 1, comprehension of nouns, active sentences, and passive sentences was assessed with a looking-while-listening task. Boys with fragile X syndrome demonstrated comprehension of active, but not passive sentences. Study 2, assessed extensions of syntactic knowledge for interpreting sentences containing novel verbs in an infrared eye-tracking preferential looking task. Boys with fragile X syndrome showed a somewhat different pattern of performance than boys with ASD. Study 3, was designed to examine variability in language development in terms of individual cognitive and biological characteristics, with an emphasis on the role of phonological memory. Cognitive processing abilities were found to be significant predictors of language outcomes. Results are discussed with reference to the theoretical importance of language learning mechanisms in differentiating linguistic phenotypes associated with neurodevelopmental disorders. [The dissertation citations contained here are published with the permission of ProQuest LLC. Further reproduction is prohibited without permission. Copies of dissertations may be obtained by Telephone (800) 1-800-521-0600. Web page: http://www.proquest.com/en-US/products/dissertations/individuals.shtml.]