In 2010, the Wolf Trap Foundation for the Performing Arts, Institute for Early Learning Through the Arts, was awarded an Arts in Education Model Development and Dissemination (AEMDD) grant to develop, implement, and disseminate a research-based program of professional development (PD) that equips prekindergarten and kindergarten teachers to infuse mathematics instruction with arts instruction in their classrooms. The PD includes summer institutes and classroom-based residencies in which music, dance, and drama performing artists work with teachers in teams. This instructional approach is often called arts integration. American Institutes for Research (AIR) conducted an evaluation of the four-year grant from 2010-2014, examining the implementation of the PD and assessing its impact on teacher practices and student mathematics knowledge. This article reports on the experiences of the elementary school teachers and Wolf Trap teaching artists in the first cohort of participating schools during 2011–12 and 2012–13, drawing on data from a variety of sources (PD observations, residency artifacts, artist interviews, and teacher surveys). We find that the Wolf Trap PD program demonstrates features of effective PD. It is classroom-based, intensive, and focused on what teachers and students need to know to teach and learn mathematics. It is aligned with district standards and offers many opportunities to teachers for active learning. The Wolf Trap PD program delivered preparation to teachers to infuse performing arts-based strategies into their mathematics instruction, starting in the PD institutes and then continuing in the residencies and did so with fidelity to the planned model. Wolf Trap used several approaches to optimize fidelity: a planning year and practice sessions with teaching artists, consistent use of local content experts, and materials structured to reflect the concepts and approaches used in both institutes and residencies. The article concludes with suggestions for practitioners and questions for further research.