Results from the first extensive evaluated project of this kind in Finland, the Lahti Project, and program and evaluation plans for a new project in the Helsinki metropolitan area are described. It is argued that, in the present Finnish context, there is a need for detailed theoretical and well-measured evaluation on why results of community-based prevention are or are not achieved. The author pays specific attention to elements of community research, content analysis, and basic areas to evaluated in assessing a program's effectiveness. She concludes that community-based prevention of alcohol and drug problems is likely to combine the resources of well-organized welfare services in municipalities with grass roots voluntary work and that a successful program will not be horizontal or vertical but rather mixed. A glossary of evaluation terms is included. 6 references and 1 figure