Well-established theoretical models and a body of empirical research elucidate the critical role of content knowledge in comprehending texts. However, the potential of supporting knowledge in service of enhancing linguistic and reading comprehension has been a relatively neglected topic in the science of reading. In this article, we explicate why knowledge building in English Language Arts instruction (i.e., content-rich ELA instruction) can support language and content knowledge, leading to better linguistic and reading comprehension, based on theoretical arguments and empirical studies. In particular, we review the evidence on this claim, paying special attention to experimental trials conducted in K-2 settings. We also share preliminary findings from a novel intervention study testing one instantiation of a widely used content-rich ELA curriculum. Whereas this growing literature base demonstrates evidence of promise, further rigorous trials are needed to examine the efficacy of this integrative approach to teaching reading for understanding. [This paper was published in "Reading Research Quarterly" v55 spec iss 1 pS99-S107 2020.]