This paper arises from a two-year project: "Teachers as writers: Transforming professional identity and classroom practice'" and draws on self-efficacy questionnaire data collected at the beginning and end of the project and interview data from five participating high-school teachers who were also co-researchers in the project. While the quantitative data reveal positive and frequently significant effects in terms of self-efficacy as writers and teachers of writing, the interview data show how changes in self-efficacy were moderated by the way individual teachers cognitively processed "source" data. The paper highlights the complex nature of domain-specific self-efficacy beliefs. It also teases out some implications in respect of the design and implementation of professional development based on Writing Workshop principles and practices. (Contains 4 tables.)