Human life is full of joint action and our achievements are, to a large extent, joint achievements that require the coordination of two or more individuals. Piano duets and tangos, but also complex technical and medical operations rely on and exist because of coordinated actions. In recent years, research has begun to identify the basic mechanisms of joint action. This work focused on simple tasks that can be performed together without practice. However, a striking aspect of human joint action is the expertise interaction partners acquire together. How people acquire joint expertise is still poorly understood. JAXPERTISE will break new ground by identifying the behavioural, cognitive, and neural mechanisms underlying the learning of joint action. Participating in joint activities is also a motor for individual development. Although this has long been recognized, the mechanisms underlying individual learning through engagement in joint activities remain to be spelled out from a cognitive science perspective. JAXPERTISE will make this crucial step by investigating how joint action affects source memory, semantic memory, and individual skill learning. Carefully designed experiments will optimize the balance between capturing relevant interpersonal phenomena and maximizing experimental control. The proposed studies employ behavioural measures, electroencephalography, and physiological measures. Studies tracing learning processes in novices will be complemented by studies analyzing expert performance in music and dance. New approaches, such as training participants to regulate each other’s brain activity, will lead to methodological breakthroughs. JAXPERTISE will generate basic scientific knowledge that will be relevant to a large number of different disciplines in the social sciences, cognitive sciences, and humanities. The insights gained in this project will have impact on the design of robot helpers and the development of social training interventions.