This article reports on how teacher educators from a university, acting as facilitators, supported teachers in conducting a school-based action research project as a practice of professional development in the context of reform in language assessment in Hong Kong. In particular, the article problematises how the facilitators and teachers negotiated and managed identities whilst being engaged in a collaborative action research project. Data were collected from semi-structured interviews. Critical discourse analysis was used to examine the textual data. A key finding was that identities were neither fixed nor finite in the context of collaboration, but were negotiated within and against a range of contextually salient discourses. A major contribution of the article lies in its examination of the complexities of negotiating identities when educators from two different institutional cultures collaborate. The article suggests that collaboration has to be understood within broader sociocultural contexts to identify the interplay of forces that shape relations, identities, and practices constructed.