In this article the authors describe their efforts to support teachers' development of deep, connected understandings of school mathematics and to find ways to make use of the teachers' learning as a context for subsequent mathematical and pedagogical development. The authors detail the evolution of their model for Online Asynchronous Collaboration (OAC), which is grounded in current research in mathematics teacher education and that takes advantage of the collaborative nature of the Internet and the permanent nature of the online interactions. They believe that OAC is a promising practice in teacher education and will discuss both the rationale for their design decisions and provide examples of teachers' activity within OAC. (Contains 2 figures and 2 footnotes.)