Professional development (PD) is seen by a broad cross-section of stakeholders as essential for instructional improvement and student learning. And yet, despite deep investments of time and money in its design and implementation, the return on investment remains low and teachers’ subjective assessments about PD in general are ambivalent at best. In this study, I asked 25 teachers to identify and reflect on their most powerful professional learning experiences and considered what these experiences might suggest toward improving PD design, policy, and research. Observing tremendous variation across learning experiences, I used the analytical lens of professional identity to argue that learners may be uniquely disposed toward learning experiences that contribute to or emerge from their professional identities. Adopting a comparative case study approach, I described three distinct learning affinities related to professional identity and professional development – an affinity for the what (subject content), the who (performative pedagogy), and the with whom (communities of practice) – and consider how a policy environment that integrated an understanding of learning affinities could, over time, improve professional development and transform the teaching profession.