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. The authors explicate why knowledge building in English language arts instruction (i.e., content-rich instruction) can support language and content knowledge, leading to better linguistic and reading comprehension, based on theoretical arguments and empirical studies. In particular, the authors review the evidence on this claim, paying special attention to experimental trials conducted in K-2 settings. The authors also share preliminary findings from a novel intervention study testing one instantiation of a widely used content-rich English language arts 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.