Three behaviors fit the article's definition of risk-taking behavior: sexual behavior, substance use, and motor or recreational vehicle use. Sexual activity has increased dramatically among adolescents. In a 1983 survey, 77.9 percent of males and 62.9 percent of females had experienced intercourse by 19 years of age. High rates of substance use during adolescence have been documented in national surveys since 1975. Alcohol is the most commonly used substance; daily use of alcohol in 1988 remained high at 4.2 percent, with 34.7 percent of high school seniors indicating they had five or more drinks in a row in the previous 2 weeks. Unintentional injuries are the primary cause of mortality in adolescents, accounting for 60 percent of deaths. Motor vehicle injuries are responsible for 80 percent of these deaths. Risk behaviors tend to be associated with each other in predictable ways; for example, substance abuse is positively correlated with the early initiation of sexual behavior, and the association between alcohol use and unintentional injury is well-established. Adolescence is a developmental period in which significant biological and psychological changes occur. Further, major maladaptive and developmentally inconsistent environmental changes occur in the school system. The impact of puberty and its timing appear to have the greatest effects in the areas of self-concept, developmental needs, school performance, and environmental responses. A causal model of adolescent risk-taking behavior is proposed that illustrates how the timing of biological maturation may affect specific psychosocial changes and the onset of risk-taking behavior. The model suggests that the timing of biological maturation directly influences four psychosocial factors; cognitive scope, self-perceptions, perceptions of the social environment, and personal values. 77 references and 2 figures