We examined theoretical issues concerning the development of reading fluency and language proficiency in 390 English Language Learners (ELLs,) and 149 monolingual, English-as-a-first language (EL1) students. The extent to which performance on these constructs in Grade 5 (i.e., concurrent predictors) contributes to reading comprehension in the presence of Grade 2 autoregressors was also addressed. Students were assessed on cognitive, language, word reading, and reading fluency skills in Grades 2 and 5. In Grade 2, regardless of language group, word and text reading fluency formed a single factor, but by Grade 5 word and text reading fluency formed two distinct factors, the latter being more aligned with language comprehension. In both groups a substantial proportion of the variance in Grade 5 reading comprehension was accounted for uniquely by Grade 2 phonological awareness and vocabulary. Grade 5 text reading fluency contributed uniquely in the presence of the autoregressors. By Grade 5 syntactic skills and listening comprehension emerged as additional language proficiency components predicting reading comprehension in ELL but not in EL1. Results suggest that predictors of reading comprehension are similar but not identical in ELL and EL1. The findings point to a more nuanced and dynamic framework for understanding the building blocks that contribute to reading comprehension in ELLs and EL1s in upper elementary school. They underscore the importance of considering constructs such as vocabulary, whose role is stable, and other components of language proficiency and reading fluency whose role becomes pivotal as their nature changes.