This article presents an in-depth study supporting the Scientist in the Classroom Partnership (SCP) program as a model of effective professional development (PD) for K-12 teachers. The goal of this study was to determine the core and structural features of the SCP program necessary for effective teacher PD. The SCP partners a classroom teacher with an STEM graduate student or postdoctoral fellow to co-teach 1 day per week during the academic year. During the summer, the teacher and fellow work together in a 10-day workshop to build the partnership, learn inquiry-based pedagogical strategies, and develop curriculum. Teacher participants from 9 years of the program were surveyed to determine the impact on the participants. In addition, teachers attended a 1-day retreat that allowed for data collection during storytelling and focus-group sessions. Likert surveys were analyzed, and the storytelling and focus-group data were coded for analysis. Five core and three structural features emerged from this analysis. The core features included gains in discipline and pedagogical content knowledge, inquiry strategies, and renewal of teaching. The structural features included the duration of PD (over 200 h), PD focused on specific grade levels, and the use of multiple types of PD approaches.Abbreviations: Science, Technology, Engineering, and Mathematics (STEM)