Epidemiological data on young people’s mental health are fragmented. The use of standardized tools in self-report surveys makes it possible to address certain mental health issues and study their associated factors, even though these tools have numerous limitations. Globally, despite significant variations in results—largely due to differences in the methodologies used in the studies—the data show that approximately one-quarter of young people have experienced a mental disorder in the past twelve months. The observed variations are sometimes due to genuine cultural differences. Anxiety and depressive disorders, as measured by standardized tools outside of a clinical setting, are the most common conditions among adolescents. The recent development of simple, effective, and validated tools specific to adolescents has improved the assessment of such disorders. A scale for measuring depressive severity, the Adolescent Depression Rating Scale (ADRS), has recently been validated in French and has been included in the Ecaap survey, as well as in the ESPAD survey conducted in France in 2007.[chapter excerpt]