In this article, we address the methodological implications of analysing online discussion boards with a focus on participants' changing identities. More specifically, we propose the use of a Communities of Practice framework as a heuristic method for considering how participants' contributions to online discussion play a role in changing who they become, as opposed to what they learn. We argue that many analyses of online communication fail to take account of learning as a change in identity--what/who the learner is (becoming)--and focus mainly on the opportunity for cognitive development--what they know. In doing so, we use a single case study from a discussion board for secondary-aged students of mathematics which acts as a "telling case" before demonstrating what this might say about other contexts for online learning. We argue that this has important implications for both research into, and the design of, online education.