In this dissertation I report findings from two studies of the literacy abilities of children with Williams syndrome (WS), a genetic disorder associated with intellectual disability. I had two overarching goals: 1) evaluate the applicability of theories of reading and spelling acquisition developed based on typically developing children to children with WS; and 2) provide results that would inform the development of targeted assessments and interventions. In Study 1, individual differences in phonological awareness, visual spatial perception, vocabulary, overall intellectual abilities, and reading instruction approach (systematic phonics vs. other approaches) for sixty-nine 6 - 7-year-olds (most of whom were in kindergarten) were used to predict word-reading abilities three years later. Multiple regression analyses indicated that Time 1 reading instruction method, phonological awareness, and visual-spatial perception each explained significant unique variance in word reading at Time 2. A systematic phonics approach was associated with significantly better performance than other reading-instruction approaches. Results of a simple mediation analysis indicated that vocabulary at Time 1 indirectly influenced Time 2 word reading through its effect on Time 1 phonological awareness. In Study 2, relations between spelling, word reading, and vocabulary abilities and method of reading instruction were investigated for eighty 9 - 17-year-olds. Spelling and reading abilities were highly correlated. Students taught to read using systematic phonics instruction had significantly higher spelling scores than those taught to read using other approaches. Spelling ability contributed significant unique variance to word-reading ability, beyond the effects of phonological awareness, vocabulary, and reading instruction method. Overall, the results from Study 1 and 2 indicated that the word-reading and spelling abilities of students with WS vary widely but on average are well below the mean for same-aged children in the general population. Variations in overall intellectual ability did not play a central role in accounting for individual differences in word reading and spelling. Findings from this dissertation provide support for the universality of theoretical models of reading development developed based on typically developing children and suggest that educational approaches known to be effective for children who are having difficulty learning to read are likely to be appropriate for children with WS. [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://bibliotheek.ehb.be:2222/en-US/products/dissertations/individuals.shtml.]