There is limited research exploring attachment style and defenses in adolescents. The purpose of the current research is to explore the relationship between adolescent attachment style and development of defense mechanisms, as well as attachment style and problem behaviors. A total of 1487 students from two California high-schools completed three self-report questionnaires to establish defense mechanisms, psychiatric symptoms, and attachment style. Attachment styles characterized by a positive self-image predict greater levels of mature defense mechanisms, and lower levels of immature defense mechanisms, both in the interpersonal and intrapsychic domains. Relationships between insecure attachment styles and psychopathology were mediated by greater levels of immature defense mechanisms. These results provide initial compelling evidence that: a) attachment style is an important determinant of the type of defense mechanisms utilized by the individual to maintain psychological stability; and b) defense mechanisms serve to transmit the detrimental effects of insecure attachment style on psychological health.