The capacity of any professional development effort to achieve durable change in teacher practice is affected by a host of design factors and their precise alignment with multiple delivery strategies. However, how successful professional development (PD) programs achieve an effective mix of these factors and strategies is not well understood. The purpose of this study is to identify which design factors have been found to be most essential, and to better understand how successful PD programs effectively align their delivery strategies with these factors. Aligning evaluation data collected across three years of the Trek 21 Project, a PT3 project found effective in promoting durable change in teacher practice, with key design factors identified from a decade of PD research, a successful design structure is presented and implications for future professional development initiatives are discussed. (Contains 4 tables, 1 figure, and 4 notes.)