This report describes program effects of the Tri-Ministry Study—a school-based, longitudinal trial carried out over a 5-year period to assess the effectiveness of classwide social skills training (SS), partner reading (RE), and a combination of both (SS & RE) to reduce maladjustment among children in the primary division (up to grade 3) of Ontario schools. It also places these effects in the context of other school-based prevention studies and discusses them in view of important methodological and programmatic issues. The incremental effects attributable to the intervention programs were small and sporadic. There were statistically significant increases in prosocial behaviour observed in the playgrounds of intervention schools with no differentiation by program type. Furthermore, there was some evidence—a reduction in teacher and parent-rated externalising problems—that the combination of SS & RE and SS alone may have had modest beneficial effects. A review of nine other school-based studies, which evaluated universally delivered mental health prevention programs in general populations of students, revealed similar mixed results. There are both methodologic and programmatic issues implicated in the weak findings that have been reported to date. These issues need to be addressed to advance knowledge about the potential impact of mental-health prevention initiatives delivered universally through school-based programs. A companion paper gives the specific details on the programs, randomisation of schools, selection of subjects, measurements, and analysis.