This article presents an analytical approach for documenting the identities for teaching that mathematics teachers negotiate as they participate in 2 or more communities that define high-quality teaching differently. Drawing on data from the first 2 years of a collaboration with a group of middle school mathematics teachers, the article focuses on a critical initial condition for teachers to improve their practice--determining that the effort required is worthwhile. The results speak directly to a central issue that arises when supporting teachers' efforts to improve their instructional practices: their motivation for affiliating with a vision of teaching that involves centering instruction on student thinking. (Contains 1 table and 1 footnote.)