This study examines teachers' initial conceptions of an integrated STEM-PBL approach and their perceptions of what facilitates or hinders implementation in their schools. This study took place within the first year of a three-year STEM Professional Development (PD) program for middle school math, science and English language arts teachers. The two primary PD components were a week-long, summer institute and a school-year cross-curricular unit planning that included a lesson-study component. Qualitative methods were used to analyze teacher responses from PD evaluations, teacher reflections, interviews and program artifacts. The study provides insight into the feasibility of an integrated STEM-PBL model. The study concludes with implications for practice and a discussion of how future interdisciplinary professional development can be conceptualized.