Teacher professional development activities in the USA take many forms from half-day workshops that focus on particular topics or classroom techniques to long term course work that offers university level credit. With few exceptions, the primary goal of such activities is to enhance the teachers' classroom effectiveness and improve student achievement. In this article, we describe a professional development model that strives to provide middle school mathematics teachers with a deep understanding of the mathematics they teach, and our attempts to measure its influence on their mathematics content knowledge. (Contains 3 tables.)