This systematic review examined whether building content knowledge in science and/or social studies topics during English Language Arts (ELA) instruction enhances vocabulary and comprehension. It also described how ELA instruction has been leveraged to build content knowledge in the elementary years. A systematic search of the extant literature identified eight (quasi)experimental studies. Five of the eight studies met standards of research quality for validating causal inferences. The results of the meta-analysis of the five studies showed that supporting content knowledge in ELA strengthened vocabulary related to content being taught (effect size g = 0.29). The average effect size for comprehension was positive and significant when standardized comprehension measures and researcher-developed comprehension measures were considered together (g = 0.24). In addition, we found common and different patterns of content-rich ELA instruction regarding the use of texts, reading, writing, discussion, and/or hands-on activities.