Teachers who have participated in Summer Institutes of the National Writing Project (NWP) have often claimed "it changed my life." What do teachers mean when they say this? What does it mean to "transform" in a professional development setting, and what might researchers and professional development providers gain from an understanding of teacher transformation as a kind of teacher learning? And how, if at all, does the writing that teachers engage in at the Summer Institute matter for transformation? This article addresses such questions through a study of teacher participants in one NWP Summer Institute involving analysis of interview, observation, and textual data. The author offers a model of teacher transformation that highlights the influence of writing groups and feedback. (Contains 1 figure, 6 tables and 2 notes.)