Among all the factors that influence the success of preschool programs, none is more important than the quality of the teaching workforce. The design and delivery of effective approaches to professional development (PD) are central to the support of the early childhood education workforce. In this article, we provide a model outlining the PD features that help to ensure that PD is effective, as well as the program- and policy-level supports needed to implement this type of PD as a part of local, state, and federal preschool programs. Throughout the article, we summarize recent research that is refining our understanding of the characteristics of effective PD, and we draw from research as well as our own experience in working with Head Start and state preschool programs across the country to illuminate challenges and promising practices in implementing effective PD at scale. (author abstract)