Interactions that occur between teacher and student during instrumental music lessons are complex and multifaceted and embrace a full range of promotive and demotive factors that not only underpin effective learning, but also have an impact on whether children will persist with their learning long-term (McPherson, Davidson, & Faulkner, 2012). Such interactions also provide the context in which students gain a sense of personal control and autonomy over their learning (Evans, McPherson, & Davidson, 2012). In this article, we present new ways of conceptualizing autonomy by taking the teacher–student interaction as a unit of analysis to examine the processes of co-regulation of student autonomy. In Study 1, we performed an in-depth qualitative analysis of four teacher – student dyads in individual string lessons. We found large differences between these dyads in the way autonomy is co-regulated from moment to moment. These differences are theorized to be in part connected to the general need for autonomy of the student. In addition, we found that teachers have different ways of dealing with students' expressions of autonomy in lessons. In Study 2, we tested whether large moment-to-moment differences between the teacher's and student's levels of autonomy ('out-of-synch' moments) contributed to macro-level student outcomes. Here, we found a positive relation between the amount of out-of-synch per lesson and, respectively, students' motivation and progress.