Student performance in mathematics has been linked to the mathematical knowledge of the teacher. Based on this finding, a 5-day professional development module was created to improve teachers' mathematical knowledge and their understanding of number sense. We found no difference prior to the professional development in mathematical content knowledge for teaching mathematics (CKTM) between special education teachers (at the K-12 level) and general education teachers (K-6). Results revealed that participating teachers made significant gains in mathematical CKTM. Implications and recommendations for professional development in mathematics are provided. (Contains 2 tables and 7 figures.)