Research in the out-of-school time (OST) field confirms that there is a strong connection between professional development (PD) for staff and positive outcomes for youth. According to Heather Weiss, Founder and Director of the Harvard Family Research Project (HFRP), "Professional development for those who work with children and youth is fraught with challenges and ripe with opportunity – specifically, the opportunity to increase staff quality, which experts agree is critical to positive experiences for children and youth" (Weiss, 2005/2006). However, as Thomas Guskey (1998) states, "For many years, educators have operated under the premise that professional development is good by definition, and therefore more is always better. If you want to improve your professional development program, the thinking goes, simply add a day or two." Thus, although considered important for staff, OST professional development often does not benefit from adequate attention or expertise. The purpose of this document is to broadly define OST professional development and summarize promising practices in its design, implementation, and evaluation.