Professional development in teaching is a critical component of ongoing work to improve student learning outcomes in higher education, especially STEM education. While there are many large-scale professional development programs designed to help participants change the way STEM is taught, few have thoroughly evaluated the outcomes to determine whether faculty members have adopted new techniques and transferred what they learned to their teaching practice. Importantly, without substantive assessment of long-term professional development outcomes, we are left with little evidence of program effectiveness. In this article we examine the current state of professional development evaluation in STEM higher education, propose possible study design elements to use when investigating the impact of professional development on instructors, and describe a novel longitudinal research design for the evaluation of professional development activities.