The implementation quality of the Tier 1 Program (Secondary 1 Program) of the Project P.A.T.H.S. in the third year of the Full Implementation Phase was examined in this study, with classroom observations of 233 units in 157 schools. Results demonstrated that the overall level of program adherence was generally high with an average of 72.96%. A total of 13 aspects regarding program delivery were significantly correlated. Multiple regression analyses indicated that overall implementation quality was significantly predicted by student participation and involvement, classroom control, use of positive and supportive feedback, degree of achievement of the objectives, and time management. Student participation and involvement, classroom control, degree of achievement of the objectives, and time management were significant predictors of success of implementation. Copyright © 2011 by Walter de Gruyter Berlin Boston.