This case describes an ongoing effort to develop and implement an innovative developmental evaluation framework to support a community-based youth violence prevention system in Seattle, Washington, called the Seattle Youth Violence Prevention Initiative. This initiative began in 2009 in response to several gang-related homicides, and now serves approximately 1,500 youth between the ages of 12 and 17 city-wide. The Seattle Youth Violence Prevention Initiative is a model of cross-sector collaboration in which three neighborhood networks in the central, southeast, and southwest sections of Seattle coordinate efforts to keep vulnerable youth connected to school, provide them with opportunities for skills training and employment, and link them to appropriate social services. While there have been a few attempts along the way to study the impacts of Seattle Youth Violence Prevention Initiative, findings have been inconclusive. There is agreement among the practitioners and researchers involved in this effort that a more comprehensive and sustainable approach to evaluation is required. There is also agreement that any evaluation framework must prioritize service quality by building capacity within the initiative for continuous quality improvement and data-driven decision-making. This case describes the development of a research–practice partnership to support and sustain evaluation efforts using a developmental evaluation framework.