Bringing learning through play pedagogies to schools and children requires major investments in teacher professional development (TPD) that builds teachers' understanding of the value of learning through play, enables them to build and practice strategies and skills to support learning through play in the classroom, and supports their ongoing development as facilitators of this approach to teaching and learning. The LEGO Foundation funded Education Development Center (EDC) to conduct foundational research on the elements of effective TPD, the conditions that enable and support teacher implementation of learning through play pedagogies, and approaches to measuring teacher and student outcomes related to learning through play. EDC's research is guided by the following questions: (1) What are the components and characteristics of effective TPD interventions, particularly those that support learning through play pedagogies? (2) What are the enabling conditions that may moderate the impact of TPD interventions and the implementation of these pedagogies in classrooms? and (3) How do existing research and evaluation studies of interventions aligned with learning through play measure teacher and/or student outcomes? This report brings together a review of existing research, exploration of existing programs, and interviews with teachers to illuminate the potential of TPD to support learning through play as well as the challenges present in this work. The goal of this report is to inform efforts to further this work in the United States.