Adolescence is often characterized as a tumultuous time in youth development, marked by occasional mood swings and intense emotions. For some young people, however, more serious, internalizing problems such as depressive or anxious moods, negative self-perceptions and emotional distress, compromise healthy development. In this brief, What Works to Prevent or Reduce Internalizing Problems or Socio-Emotional Difficulties in Adolescents, Child Trends synthesized findings from 37 random-assignment social intervention programs designed to prevent or treat internalizing problems for adolescents.