Although collaboration is often considered a beneficial learning strategy, research examining the claim suggests a much more complex picture. Critically, the question is not whether collaboration is beneficial to learning, but instead how and when collaboration improves outcomes. In this paper, we first discuss the mechanisms hypothesized to support and hinder group learning. We then review insights and illustrative findings from research in cognitive, social, and educational psychology. We conclude by proposing areas for future research to expand theories of collaboration while identifying important features for educators to consider when deciding when and how to include collaboration in instructional activities.