This article reports the development of the first two years of a three-year project that examines the effectiveness of adult-child interactions in the early years, and tracks the development of teachers' thinking and practice as they engage in practitioner-led action research. The Adult-Child Interaction Project involves practitioners who work with children from six months to six years in one local authority in England as they aim to develop their practice. On the basis of action research cycle, the methods include systematic video observation and stimulated recall; individual, paired and collaborative reflective dialogue; project logs and critical engagement with research. A distinctive feature is continuous engagement with an early years expert as a co-researcher and reflective partner in the processes of change and development. This article focuses on the impact and effectiveness of the project methodology, specifically the strategies that support critical reflection, dialogue and change processes.