There is no shortage of recommendations regarding the use of visual representations (e.g., graphs, Venn diagrams, tree diagrams) as problem-solving tools. Moreover, research generally supports their use in problem solving. What is lacking, however, is detailed knowledge about the benefits of visualization. How do students use visual representations, problem solving. What is lacking, however, is detailed knowledge about the benefits of visualization. How do students use visual representations, what advantages do the representations provide, and how do students balance visual representations and other mathematical representations? In this talk, I will review recommendations about visualization and present the results of two recent classroom studies that seek to address these questions.