In this paper we describe the process of moving a 45-hour, mandated, in-person professional development course into a 70% online, 30% in-person format. We address various challenges that arose during the process including 1) maintaining a socio-constructivist philosophy in an online setting, 2) working within the technological limitations that result from the course being required for working teachers, and 3) responding meaningfully to participants’ online discourse given the participation of 30-60 teachers per semester. Specifically, we will describe our asynchronous online module framework that includes progressive formalization and social learning. In addition, we present quantitative and qualitative data in order to evaluate whether the course has been successful in both increasing teachers’ mathematical content knowledge and changing perceptions about the teaching and learning mathematics.