Data for the analysis came from a prospective study of social factors influencing development in the high school years. Conducted in Boston area communities, the study was based on a probability sample of 9th, 10th, and 11th graders in public schools and involved three waves covering the 1988-1990 period. Covariance structure models based on polychoric correlations among study variables revealed that friends primarily influenced the behavior of young people; estimates of such influence, however, were grossly overstated in analysis relying on respondents' perceptions of friends' behavior. Parental supervision and attachment were weakly related to subsequent delinquency and marijuana use, lending little support to control theories of deviance. Different processes accounted for similarities among members of delinquent and drug-using peer groups. Although young people were socialized into delinquent behavior by peers, both selection and socialization influences played important roles in the formation of drug-using peer groups. An appendix tabulates data on polychoric correlations among study variables 64 references, 7 notes, 4 tables, and 3 figures