Efficacy trials test whether interventions work under optimal, highly controlled conditions whereas effectiveness trials test whether interventions work with typical clients and providers in real-world settings. Researchers, providers, and funding bodies have called for more effectiveness trials to understand whether interventions produce effects under ecologically valid conditions, which factors predict program effectiveness, and what strategies are needed to successfully implement programs in practice settings. The transition from efficacy to effectiveness with preventive interventions involves unique considerations, some of which are not shared by treatment research. The purpose of this article is to discuss conceptual and methodological issues that arise when making the transition from efficacy to effectiveness research in primary, secondary, and tertiary prevention, drawing on the experiences of two complimentary research groups as well as the existing literature. We address (a) program of research, (b) intervention design and conceptualization, (c) participant selection and characteristics, (d) providers, (e) context, (f) measurement and methodology, (g) outcomes, (h) cost, and (i) sustainability. We present examples of research in eating disorder prevention that demonstrate the progression from efficacy to effectiveness trials.