This study examined how a task-focused, year-long mathematics professional development program influenced elementary school teachers' knowledge, beliefs, and practices. Participants completed 84 h of professional development over 13 months that were focused on exploring, modifying and implementing cognitively-demanding mathematical tasks. Using a multi-methods approach, teacher-participants completed pre- and post-measures of mathematical knowledge for teaching, teachers' beliefs about teaching and learning mathematics, and teachers' self-reports of enacted instructional practices. Further, three teacher-participants were randomly selected to be observed 3 times over the course of the school year. Data analyses indicated that the professional development had a statistically significant positive impact on participants' mathematical knowledge for teaching, use of student-centered instructional practices, and beliefs towards mathematics as a subject area. Further, the observed teachers enacted some high-level mathematical tasks and questions, but these were more visible at the end of the study compared to the beginning of the study. Implications for future work are also shared.