Recent research suggests that drawing activities can help students learn concepts in the science, technology, engineering, and mathematics (STEM) disciplines. In particular, drawing activities, which mimic the practices of STEM professionals, can help students engage with visual-spatial content. However, prior work has also shown that students struggle to learn from drawing activities. One major issue is that the learning processes underlying the effects of drawing activities are mostly unknown, and therefore, it is unclear how best to design effective drawing activities in STEM learning environments. To address this gap, our review of prior research investigates which learning processes may explain how drawing activities facilitate learning of STEM content. Specifically, we reviewed prior research across cognitive and sociocultural theoretical perspectives. We identified six learning processes fostered by drawing activities. Each learning process describes how drawing can change the way students interact with the content. Our review shows how instructional support for drawing activities that targets each learning process can enhance learning. Our findings have theoretical implications regarding how drawing activities have been studied and yield open questions about the mechanisms accounting for the effects of drawing activities on students' learning in STEM disciplines. Further, our findings suggest practical recommendations on how to effectively implement drawing activities that help students learn STEM content. [This is the online version of an article published in "Educational Psychology Review." For the final published version of this article, see EJ1206582.]