This chapter provides the study of how people perform and are influenced by each other when in pairs or groups has begun to receive significant attention in performance contexts. It reviews some of this literature with the aim of determining if and how partners impact each other’s learning of movement skills. Individual training sessions have typically been considered the most beneficial way to train people. There are many physical activity contexts where individuals practice simultaneously with co-learners or with more accomplished performers. It is important to study how acquired skills might be specific to the partner(s) with whom they were acquired, especially if we wish to apply this research to more experienced performers in sports. It is often the case that sports require the athlete to perform multiple skills, such as different types of serves or return strokes in tennis or different shots or passes in sports such as basketball or hockey.