This qualitative study reports on the perceptions of a group of primary/elementary teachers who are engaged in a collaborative action research project, Teachers in Action, focused on enhancing STEM teaching and learning. The teacher researchers are novice action researchers from five school districts in Newfoundland and Labrador, Canada. At the beginning of the project and in the early stages of the action research process, they were asked to share their views as they relate to professional development in STEM disciplines. The research questions that guided the study are: a) What are primary/elementary teachers' conceptions of effective professional development in STEM education? b) How do these views align with current research on what constitutes effective PD? and c) How will the approach to collaborative action research adopted in this study impact primary/elementary teachers' professional learning in STEM disciplines? This chapter focuses primarily on the first two questions. Outcomes report on teachers' perceptions of the characteristics of effective professional development in science, technology, engineering, and mathematics education; the supports needed to engage in effective professional development; and the potential successes and constraints associated with professional learning in STEM education through collaborative action research.