There is a growing need for more information regarding the early academic development of preschool-age children who are learning to speak Spanish and English. To achieve this, studies are needed that acknowledge the heterogeneity of language abilities within bilingual preschool children, and that these variations in language abilities may impact children's early academic skills. This two-study dissertation investigated how four foundational skills: narrative production, phonological awareness, letter-word identification, and early math, differed depending on the language dominance Spanish-English bilingual children possessed at preschool entry. The studies used data from a larger language, literacy, and self-regulation project entitled Tools of the Mind: Promoting ELLs' Language, Self-Regulation & School-Readiness. Participants were typically-developing children of Latino heritage recruited from early childhood centers with primarily English instruction. Performance on a standardized language battery given in English and Spanish was used to assign children to one of three language-dominance groups (i.e., stronger-English, balanced abilities, stronger-Spanish). Both studies yielded several findings that make unique contributions to research on bilingual preschoolers. The first study underscored the important relations of both lexical diversity and grammatical production abilities to bilingual preschoolers' narrative macrostructure. The second study identified specific areas of school readiness strengths for each language dominance group and identified areas that may need additional support. Implications of interest to speech-language pathologists and other early childhood professionals are discussed, including implications for assessment and differential instruction. [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.]