The 2007 Aaron Rosen Lecture was presented at the annual meeting of the Society for Social Work and Research, January 12, 2007, in San Francisco, CA. The lecture begins by describing the gap between what is known about efficacious treatments and other evidence-based practices on the one hand and the services that are provided in actual community-based practice settings on the other. To address this gap, the lecture calls for the development of a science of implementation effectiveness and describes the author's research on assessing and changing the social context of mental health and social service organizations as contributing to that effort. The findings of two national studies and one controlled clinical trial are summarized to (a) link organizational social context to service outcomes, (b) describe a new profiling system for assessing organizational social context, and (c) demonstrate how social context can be changed with planned organizational intervention strategies. (Contains 4 tables and 4 figures.)