This case study focuses on the professional learning of 38 K-9 teachers who engaged in a professional development (PD) programme in Science, Technology, Engineering and Mathematics (STEM) through collaborative action research (CAR). Through the lens of CHAT or Cultural-Historical Activity Theory, the author examined what teachers were learning, the contextual factors that influenced their learning and classroom practice, and the outcomes of this PD initiative. The changing activity system of the teachers is described, as well as how they addressed contradictions in their practice. Addressing these contradictions resulted in an expansion of their object and professional growth. The study contributes to a growing body of research in teacher education which applies CHAT as a conceptual framework to understand teacher professional learning and how interactions among activity system components influence teachers' development and practice.