This article describes a professional development course intended to improve the content understanding of middle school mathematics teachers. The design of the course included three professional learning strategies: problem solving, examination of student thinking, and discussion of research. The concepts studied in the course included multi-digit subtraction, multi-digit multiplication, operations with fractions, and concepts of area and perimeter. Results from pre- and post-tests administered to the nineteen participants indicate a significant increase in the mean score for each concept and document growth in the teachers' content understanding. In particular, their solutions moved from primarily procedural to more conceptual. Responses to an open-ended survey indicate other important aspects of the professional development. Examples of teachers' work and comments are included. (Contains 4 figures and 3 notes.)