We empirically evaluated three theoretical models—the threshold hypothesis, transfer theory, and time-on-task theory—for educating English language learners (ELLs), with a focus on the role of language factors in explaining achievement differences among ELLs. Participants were 196 sixth graders with Spanish language backgrounds who started learning English in kindergarten and then were continuously enrolled in a U.S. school. Structural equation modeling was used to estimate the extent to which Spanish and English language and literacy—skills that are emphasized differentially in competing theories for educating ELLs—predict academic achievement assessed in English. Results indicated that Spanish literacy, over and above English language proficiency, was substantially predictive of academic achievement, consistent with the transfer theory. This model was a more focused version of the threshold hypothesis, in that the weaker predictor of Spanish oral language proficiency was excluded. Time-on task theory was not supported.