This paper presents a model of supervision that examines and informs the understanding of professional development within a Professional Development School (PDS) context. It offers a theoretically-based model for supervision, drawing on three bodies of literature (supervision, teacher inquiry, and PDSs). The study constructed and explored a model for how supervisory practice might unfold over the course of a PDS year and how PDS supervision involves the interaction between multiple teacher educators. After describing the PDS at Pennsylvania State University and the State College Area School District, the paper describes the role of the Professional Development Associate (PDA), presents four phases of the PDS supervision model, and discusses the PDA's role in each phase. The phases include establishing readiness, traditional supervision, reflective supervision, and teacher inquiry driven supervision. The study found that interns progressed through the phases at different rates and that supervisors and mentors had to build a relationship of mutual trust and respect. The paper concludes with implications of the model for both supervisory practice and PDSs. (Contains 32 references.) (SM)