Empirical evidence suggests oral language proficiency is correlated with literacy outcomes; however, the relationship between oral language proficiency and literacy outcomes for English Language Learners is highly variable. As a result, the field lacks critical direction to identify children who are English Language Learners experiencing general reading difficulties or difficulty due to underlying reading disabilities. These inconsistent results may be due to the fact that published studies treat English Language Learner participants as a homogenous group. As a result, the purpose of this study was to identify unobserved heterogeneity in language acquisition for children who are Spanish-speaking English Language Learners. Latent Transition Analysis was used to model meaningful patterns of acquisition in bilingual oral language proficiency. Spanish-speaking children (N = 500) in the 1st, 2nd, and 3rd grade during Year 1 (2009-2010) were tested during the Fall of each school year, for a total of three assessment waves from 2009-2012. The extended bilingual (English/Spanish) assessment battery tested language, reading, and cognition abilities on both norm-referenced and experimental measures. All testing was counterbalanced for language presentation and test order during each assessment wave. Examination of fit statistics combined with substantive theory suggested four meaningful classes of bilingual oral language proficiency emerged and remained consistent across time points. Further examination of the transitional probabilities indicated overwhelming stability for language acquisition; however, instances of developmental progress did emerge. Adding time-varying covariates of phonological processing abilities in English did not significantly change the 4-class solutions derived at each time point. Estimated means predicting distal reading comprehension outcomes from year 2 class standing suggest children's class standing was a significant predictor of reading comprehension abilities in English. Overall, findings provide unique contributions to shape educational policy, classroom practices, and the field of research for English Language Learners. [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.]