Higher education institutions prepare future faculty members for multiple roles, including teaching. However, teaching professional development programs for graduate students vary widely. We present evaluation data from a high engagement program for STEM doctoral students. We analyzed the impact on three cohorts of participants over three academic years and identified the components most influential upon their teaching professional development. Participants found the year-long teaching assessment project and the disciplinary and reflective focus instrumental for improving their knowledge of teaching and learning. We recommend these components for the design of other such high-engagement programs.