Despite the impetus from professional organizations for science and mathematics integration, few teacher training programs provide opportunities for teachers to develop the knowledge and skills in effective content integration. This study reports on the impact of a graduate course on in-service middle school, math, and science teachers' professional knowledge for content integration and their integration practice. This course model draws on the extant research on science and mathematics integration. Quantitative and qualitative results suggest the course increased teacher professional knowledge and improved integration practice. Integration practice was evaluated utilizing a project-based measure: the Mathematics and Science Integration Rubric (MASIR). Results indicate that although the integration practice and teacher conceptions of integration moved toward a more coherent application of integrated instruction, the combination of content knowledge and teacher feedback should be targets of extended professional learning. Implications for broader professional development issues for science and mathematics teachers are discussed.