The present study tested the postulation that "knowledge begets reading, which begets knowledge." Using Random Intercepts Cross-Lagged Panel Models (RI-CLPM), we analyzed a U.S. nationally representative data set to examine the directionality and magnitude of the longitudinal relation between domain knowledge (operationalized as science domain knowledge) and reading throughout the elementary years (from kindergarten to fifth grade), while accounting for important covariates, such as working memory, cognitive flexibility, English language proficiency, basic literacy skills, and demographic information. Moreover, we conducted multi-group RI-CLPM analyses to examine whether language status (being bilingual or monolingual) moderates the longitudinal relation between domain knowledge and reading. The results showed that the relation between domain knowledge and reading is bidirectional and positive throughout the elementary years, providing empirical evidence that domain knowledge and reading may mutually enhance with each other. In addition, language status did not moderate the relation between domain knowledge and reading, suggesting that the directionality and magnitude of the relation were similar between bilingual and monolingual students. Taken together, the results have important implications for integrating content knowledge and English language arts core instruction in elementary grades. [For the corresponding grantee submission, see ED623594.]