This chapter describes the empirical measures of musical ensembles—two or more individuals engaged in a musical task. The majority of empirical studies of ensemble performance to date have focused on Western tonal classical music and on small ensembles. Empirical measures of the temporal dynamics underlying ensemble music performance have focused predominantly on dyadic interactions and have uncovered the influence of both intrinsic and extrinsic factors that influence synchronization. Additional studies examined how individuals perform with a computer-generated performance or with a recording. The chapter focuses on the natural case of human ensembles, which permits temporal adaptation among all performers. It reviews empirical findings of extrinsic and intrinsic factors of temporal coordination in ensemble music performance. The chapter also focuses on mathematical theories of temporal coordination among performing musicians. Intrinsic factors associated with spontaneous coordination also play a role in temporal coordination between performing musicians. The chapter discusses future directions toward unifying models of temporal coordination in ensemble music performance.