The last three decades of work in cognitive science have challenged the idea that thinking occurs entirely in the head, claiming instead that cognition is embodied, embedded, extended, and enactive. The claims of 4E cognition challenge the dominance of computational approaches to cognition, and music scholars have explored Gibson's notion of affordances to propose a new understanding of musical performance as primarily grounded in action. This Element draws from paradigms such as enactive cognition, cybernetic and systems-theoretical approaches, phenomenological perspectives on practice, Gibson's theory of affordances, and aspects of the author's own practice as a multi-instrumentalist to consider cases of how the interface between musician and instrument influences performance.