In teams, some combinations of people work together better than others. A large body of literature with a rich history suggests that the configuration of team member attributes, called team composition, has a fundamental influence on teamwork. Team composition shapes the emergence of affective states, behavioral processes, and cognitive states (the ABCs of teamwork), which ultimately affect how teams meet their objectives. The purpose of this article is to describe what is known about team composition and its influence on the ABCs of teamwork. We discuss what team composition is, and why it is important. We then describe key discoveries related to how team composition shapes the ABCs of teamwork. Building on what we know, we outline important directions for future research.