Though no clear link between professional development and youth outcomes has been established in either the in-school or the out-of-school time (OST) literature, a research-based consensus establishes the characteristics of high-quality professional development: It is sustained over a period of time, coherent, content focused, and based in a community of learners. These guidelines can inform OST funders and program directors as they decide how to invest limited resources in the OST workforce. Empirical research still needs to be conducted to investigate the link between high-quality professional development and youth outcomes. An annotated bibliography is included. (Contains 2 footnotes.)