Between 10% and 20% of youngsters suffer from psychological problems serious enough to warrant psychological treatment (Carr, 1999). There is now an impressive body of research which shows that psychological interventions for many of these problems are highly effective (Carr, 2000). However, for all who work in child and adolescent health, education, and social services a central concern is how these psychological problems might be prevented in the first place. Our book aims to answer this question. Our answer is based on a thorough, critical review of rigorously conducted prevention programme evaluation research.