Facilitating schools to develop more collaborative cultures is seen as one of the major challenges in promoting teachers' continuing professional development. This article recounts how a fifteen-month project designed to promote greater collegial learning and professional development emerged and developed within one school. Evidence suggests that a model that is voluntary and that involves teachers observing each other's classrooms and sharing insights is both effective and sustainable. Trust, supportive school leadership and external facilitation are also identified as important components in the realization of this model of teacher development. (Contains 7 notes.)