Among younger people in many high-income countries, mental ill-health, which includes depression, aggressive behavior, feeling down, and alcohol and drug abuse, is one of the greatest health problems. Since most young people attend school, there are grounds for pursuing the prevention of ill-health in the educational arena. A set of techniques, named social and emotional learning (SEL), based on cognitive and behavioral methods, is available to teachers to train students to improve self-control, social competence, empathy, motivation and self-awareness. SEL programs have their underpinnings in the theories of cognitive development and social learning, and in application of the ideas of risk and protective factors. The primary aim of this dissertation is to describe and evaluate, in a real-life setting, the impacts of a Swedish program derived from SEL, called social and emotional training (SET), on various mental-health outcomes. Such programs have been shown to have favorable effects in the international literature, but have not been tested before in Sweden. Sub-aims were to investigate whether there were outcome differences between subgroups, and to assess the development of an instrument for the measurement of social emotional maturity. The evaluation was performed in two experimental and two control schools (41 and 20 classes, respectively) in Botkyrka Municipality in Greater Stockholm. A variety of statistical analyses were applied to the data collected: two repeated-measures cohort analyses, with rather different designs, to measure changes over two and five years; latent-class analysis to examine variability and substance use; and, latent growth curve modeling with full maximum likelihood estimation to scrutinize our earlier findings . On the social and emotional variables, the impact of SET was found to be generally favorable. After five years, the impact of SET was found to be greater for internalizing than for externalizing problems, but no impact on socia