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. The purpose of the current paper is not to provide an extensive review of the multiple literatures related to collaborative learning. Instead, we aim to bring together complementary insights from research in cognitive, social, and educational psychology on the factors and processes that affect collaborative success and failure that have not been brought together in past work. Our central thesis is that collaborative learning is an important educational practice that can be effective when particular cognitive and social processes are supported. We describe the mechanisms hypothesized to inhibit or support collaborative success in the context of recent theories of collaboration. We then review representative findings from two rich research traditions that have differed in how they assess collaborative outcomes by comparing the group to the average individual or to nominal groups (i.e., the group's predicted potential).