Citywide implementation of project-based learning highlights the benefits--and the challenges--of promoting exemplary practices across an entire out-of-school time (OST) network. In summer 2009, the City of Philadelphia and its intermediary, the Public Health Management Corporation (PHMC), introduced project-based learning to a network of more than 180 OST programs. Use of project-based learning is now required of all city-funded OST programs that are managed by PHMC. This article describes Philadelphia's systemwide approach to project-based learning. First, the authors review the scholarly literature to define the strategy and discuss its outcomes. Next, they describe Philadelphia's systems approach to project-based learning in OST and outline its successes. Finally, they discuss the challenges presented by a systems approach to implementation, offering recommendations to other cities and their intermediaries that wish to implement project-based learning on a systemwide basis. (Contains 1 table.)