Our primary purpose in this article is to propose an interpretive scheme for analyzing the identities that students develop in mathematics classrooms that can inform instructional design and teaching. We first introduce the key constructs of normative identity and personal identity, and then illustrate how they can be used to conduct empirical analyses. The case on which the sample analysis focuses concerns a single group of middle school students who were members of two contrasting classrooms in which what it meant to know and do mathematics differed significantly. The resulting analyses document the forms of agency that students can legitimately exercise in particular classrooms, together with how authority is distributed and thus to whom students are accountable, and what they are accountable for mathematically. In the final section of the article, we clarify the relation of the interpretative scheme to other current work on the identities that students are developing in mathematics classrooms. (Contains 1 figure and 15 footnotes.)