<p>This paper provides a vignette of one teacher’s identity transformation from a temporary teacher of mathematics to a professional mathematics teacher (with a long-term trajectory within the profession) during the course of his participation in an In-Service Education and Training (INSET) program. Thus the paper elaborates, from Wenger’s (1998) social practice perspective, the role of identity (one of Wenger’s four learning components) in teacher learning and teacher retention within the profession.</p><p>The vignette provides textured insight into the way in which Sam’s identity transformation was pivotal in his learning through participation within INSET and other overlapping mathematics education communities. Furthermore, I argue that identity (learning as becoming) was a key component of learning that kept Sam in the profession of mathematics teaching. From Sam’s vignette I argue that positive identity transformation should be one of the most important intended learning outcomes of INSET and as such needs deliberate and focused attention in the design and implementation of teacher education programs. I therefore conclude the paper with some implications of Sam’s vignette for the design of mathematics INSET programs.</p>