In-service professional development is important for improving teaching. However, little research has examined how the roles, beliefs, and backgrounds of the individuals providing professional development can best be leveraged to create effective professional development programmes. A particularly understudied group are community-based On-Site Teacher Educators (OSTEs) who can serve as the bridge between university-based faculty and school employees. OSTEs are aware of the realities of classroom spaces, yet work outside of them, giving them a hybrid capacity to support teacher learning. In this exploratory qualitative multiple case study, the perspectives and practices of three OSTEs are examined as they supported elementary science teachers (n = 119) in multi-year professional development. Findings indicate that OSTEs have strong alignment between their professional development practices and their beliefs about knowledge and learning. OSTEs revealed that they are introspective about managing the gap between university faculty and classroom teachers. Cross-case analysis revealed that OSTEs identify a tension between affirming teachers as professionals and productively challenging teachers' assumptions. To address this, they use modelling, co-teaching, and dialogue inside a stance of teaching as intellectual work. In sum, this research suggests that professional development provided by OSTEs can lead to transformative change in schools.