The main finding of the study is that a positive self-concept is important for good mental health and overall positive development in adolescence, especially for girls, as it offers protection against the development of common problem behaviors (aggressive and delinquent/antisocial behavior) during this period. A negative self-concept, on the other hand, particularly for girls, is a risk factor of internalized problems, such as anxiety and depression. Self-control had a direct effect on externalizing behavior only for boys. Adolescents who were 15 and 16 years old showed a stronger link between a negative self-concept and externalizing problem behavior than younger and older adolescents. Internalizing problems such as anxiety, depression, complaints about physical problems, and being withdrawn influenced problems with aggressiveness, delinquency, and rule-breaking behaviors such as truancy, lying, and the use of alcohol. The findings indicate the importance of promoting a positive self-concept in adolescents in the various psychosocial contexts that compose their experiences. Study participants were 277 adolescents (131 boys and 146 girls) ages 12 to 18 years old, who were selected from schools in varying socioeconomic districts. Self-concept was measured with the Swedish Long Form Version of the Structural Analysis of Social Behavior questionnaire. The Youth Self-Report (YSR) checklist measured self-reported competencies and problems. The YSR contains the following subscales: Withdrawn, Somatic Complaints, Anxious/Depressed, Social Problems, Thought Problems, Attention Problems, Delinquent Rule-Breaking, Aggressive, and Self-Destructive/Identity Problems. Links between self-concept and internalizing and externalizing behaviors were tested with path analysis. 2 tables, 3 figures, and 83 references