This meta-analysis examined if students writing about content material in science, social studies, and mathematics facilitated learning. Studies in this review were conducted with students in Grades 1 to 12 in which the writing-to-learn activity was part of instruction. As predicted, writing about content reliably enhanced learning (effect size = 0.30). It was equally effective at improving learning in science, social studies, and mathematics as well as the learning of elementary, middle, and high school students. Writing-to-learn effects were not moderated by the features of writing activities, instruction, or assessment. Furthermore, variability in obtained effects were not related to features of study quality. Directions for future research and implications for practice are provided.