Diagrams and visualizations can be seen throughout educational materials, such that their presence is almost ubiquitous. The assumption is that including visualizations in lessons will lead to greater learning. In this paper, I review the psychological and educational literature to examine how the presence of visualizations influences learning in STEM fields. In particular, I examine how the effectiveness of a visualization depends on characteristics of the visualization, the learner, and the context. Characteristics of the visualization are important insofar as they highlight relevant information and reduce extraneous details. Learner characteristics influence how people learn with visualizations, but more work is needed to isolate these relations. The characteristics of the context are a critical but underexplored area in multimedia learning. This review highlights that visualizations are not universally helpful for learning, but rather their effect depends on a myriad of factors...