Noting that prevention programs seldom work well, primarily because they are not implemented as required by the underlying program theory, this article summarizes literature suggesting that features of the implementing organization and its ecological context are related to the strength and fidelity of implementation. Schools are used as a focal point for studying the organizational correlates of implementation strength, and evidence is summarized concerning the demographic and geographic location of schools possessing characteristics likely to hinder implementation. The article demonstrates how these factors may undermine a potentially strong prevention program, using as an example an urban school-based prevention program. The school is described, including the history of the implementation, outcomes, and reasons for failure. The reasons for failure are particularly highlighted, and include problems with: (1) training and technical assistance; (2) outsiders; (3) leadership; and (4) organizational capacity. The article concludes by noting that high-quality programs are planned, coordinated, and ongoing; include multiple interventions; rely on behavioral science technology; and integrate education, skills development, and motivation on a range of health problems and issues. The following recommendations are offered: identification of meaningful measures of quality school health education programs, acquiring information about causal factors that determine level of implementation, formulating theory to strengthen intervention practices, valid assessment instruments, and appropriate staff training. Contains approximately 75 references. (SD)