This paper describes a model for studying collaborative knowledge building (CKB) as a group activity. We integrate the model described by Stahl (2000a) with an analysis based on the principles of cultural historical activity theory (CHAT), using the analysis of a self reflective case study to guide the description. The concept of the CKB activity system is developed, and the role of contradictions in CHAT is described. The case is then analyzed to show how the model explains collaboration in practice. The final model includes two additional cycles representing the role of reflective practice in CKB. The new model of CKB processes combined with the concept of the activity as the unit of analysis and the tools of CHAT provides an efficacious way of investigating collaborative knowledge building.