The research objectives were to identify the middle level mathematics teachers' conceptions of mathematics teaching and learning and ascertain changes in their conceptions and instructional behavior at the end of a three-year professional development (PD) project. These changes were studied through the qualitative analysis of their reflective journals and observations of their teaching to determine the extent to which their conceptions and their instructional behaviors exemplified the PD vision. The results include the teachers' conceptions of their own learning being transformed into their conceptions of how middle level students learn mathematics and the expectations of the PD project; understanding mathematical concepts instead of memorizing rules, solving realistic application problems instead of practicing routine procedures, and students exploring and investigating instead of teachers lecturing. Supported by the evidence in their journals and teaching observations, the reflection process narrowed the gap between their teaching practice and the PD vision of reform. The results imply that the PD project focus should be at least considered for future programs; 1) strengthening teachers' mathematical content knowledge, 2) building proficiency in the use of effective teaching strategies, 3) developing mentor relationships between middle level teachers and university faculty, and 4) supporting teachers in the selection and implementation of reform curricula.