In the first part of the paper I analyze a data set on teenage behavior. The data is a sample of high school students in the Netherlands, and contains information on teenage time use, income, expenditures, and subjective measures of well-being and self-esteem. As all students in a sampled class are interviewed in principle, the data set has rich information on the behavior of potentially important peers of each respondent. I estimate models to assess (bounds on) the magnitude of endogenous social interactions. For some types of behavior (e.g. truancy, smoking, pocket money, alcohol expenditures) endogenous social interactions within school classes are strong, for other behaviors they are moderate or unimportant. Within-gender interactions are generally stronger than interactions between boys and girls, with some intriguing exceptions. In the second part of the paper I discuss a number of theories that might help to understand the empirical patterns. Key concepts in the discussion are interdependent preferences, endogenous social norms, identity, and intergenerational interactions.