Research Goes to School (RGS) is a professional development program that focuses on high school teachers of science, technology, engineering, and mathematics (STEM) subjects. This collective case study examines RGS participants and their use of project-based learning (PBL) as they implemented curricular units that they developed at the RGS summer workshop. Based on the analysis of the data from the observations, the RGS participants exhibited partial fidelity of implementation to the features of PBL. Analysis of the data from the interviews indicated that participants were aware of features of PBL that they were not able to fully implement. Participants also identified several supports, particularly from the RGS program such as being able to order materials to implement their units, as well as supports that were specific to their teaching contexts. The findings suggested that the professional development program had some positive if limited influence on teachers' instructional practices. This study highlights the need for professional development to enhance teachers' content knowledge as well as their knowledge of pedagogy and to promote collaboration between teachers and professional development providers, particularly once teachers are in their classroom contexts.