Teachers of middle school mathematics should have a deep conceptual understanding of the elementary mathematics taught in middle school, should possess the mathematics knowledge for teaching that is required to effectively teach mathematics in middle school and should have the ability to effectively teach mathematics to, and enhance the maths self-efficacy of, a culturally and socially diverse middle school student population. The challenge is to design effective professional development activities that enhance these desired attributes in both in-service and pre-service middle school mathematics teachers. This article reports on an attempt to design and evaluate one such activity that focuses on mathematics knowledge for teaching and self-efficacy building. (Contains 2 tables.)