The United States has engaged in intensive reform efforts in school mathematics education in the decade since the publication of the National Council of Teachers of Mathematics' Standards for School Mathematics. This context has generated interest in how to best develop reform-oriented teachers. In this article the authors describe their thinking towards the development of a framework for the qualitative study of secondary school mathematics teachers' learning to teach through practice. Their need for such a framework occurred in the context of their work with teachers who during their teacher preparation program participated in multiple communities where individuals shared a common goal of reforming instruction. The framework combines the following theoretical constructs: (a) Shulman's heuristic of important teacher knowledge, (b) Wenger's conception of learning as identity development in social situations, and (c) cognitive notions of thinking as residing in one's head. (Contains 1 table, 3 figures and 5 notes.)