This article reviews several classes of theories to elucidate the relationship between adolescent cigarette smoking and friends' cigarette smoking. Perceived influence theories hinge upon an adolescent's perception of friends' smoking behavior. External influence theories are those in which friends' smoking behavior overtly influences adolescent smoking. Group level theories examine how differences at the level of subculture, gender, and race/ethnicity influence the relationship under study. Network theories are also discussed. A model integrating relevant theories into a longitudinal model representing friend influences on adolescent smoking is presented, along with implications of the results presented for adolescent tobacco prevention programs.