Primary/elementary teachers are uniquely positioned in terms of their need for ongoing, science-focused professional development. They are usually generalists, having limited preparation for teaching science, and often do not feel prepared or comfortable in teaching science. In this case study, CHAT or cultural-historical activity theory is used as a lens to examine primary/elementary teachers' activity system as they engaged in a teacher-driven professional development initiative. Teachers engaged in collaborative action research to change their practice, with the objective of making their science teaching more engaging and hands-on for students. A range of qualitative methods and sources such as teacher interviews and reflections, teacher-created artifacts, and researcher observational notes were adopted to gain insight into teacher learning. Outcomes report on how the teachers' activity system changed as they participated in two cycles of collaborative action research and how the contradictions that arose in their activity system became sources of professional growth. Furthermore, this research shows how the framework of activity theory may be used to garner insight into the activity and learning of teachers as both their professional activities and the context change over time.