The uncertainties of looming adulthood, nostalgia for childhood, and a general malaise explain the crisis of adolescence. Rebellion, conflict, occasional failure at school or in society, and at-risk behaviors are not always signs of future psychiatric illness. In contrast, the physician must be in a position to identify tell-tale signs such as dysmorphophobia, existential anxiety, a feeling of emptiness, and school or social breakdown. Most psychiatric disorders that begin in adolescence are only diagnosed several years after onset. Yet early diagnosis is of utmost importance, as treatment becomes less effective and the long-term prognosis worsens with time. Suicide is the second cause of death during adolescence. All signs of suicidal behavior require hospitalization and evaluation in a psychiatric unit. Antidepressants may be necessary in adolescence. The recent controversy concerning a possible increase in the suicidal risk during antidepressant treatment should not mask the fact that the real public health issue is depression, and not antidepressants. Eating disorders are especially frequent among adolescent girls; it is important to identify psychiatric comorbidities such as schizophrenia, depression and obsessive-compulsive disorders, and to assess the vital risk. Illicit drug and alcohol consumption are frequent during adolescence; for example, close to half of all French adolescents have tried cannabis at least once. Once again, it is important to detect psychiatric comorbidities in substance-abusing adolescents. Phobia is an underdiagnosed anxiety disorder among adolescents; it may become chronic if proper treatment is not implemented, leading to suffering and disability. Finally, two major psychiatric disorders--schizophrenia and bipolar disorder--generally begin in adolescence. Treatment efficacy and the long-term prognosis both depend on early diagnosis. Treatment must be tailored to the individual patient. "Borderline" states are over-diagnosed, hindering more precise diagnosis and delaying appropriate treatment.