Presentation of the results of the "Analysis of Young People's Sexual Behaviors (ACSJ)" survey, which is part of a system for collecting data on the general population with the aim of preventing AIDS among 15- to 18-year-olds, based on a better understanding of behaviors that carry a risk of HIV infection. The sample included an overrepresentation of apprentices (ages 15–19), who are sexually active earlier than high school students but less informed about the risks. Two preliminary surveys were conducted among high school students in the Paris region and apprentices in the Toulouse metropolitan area, at their training facilities. The main survey was conducted from January to March 1994. A total of 6,445 questionnaires were collected. The questions avoided any reference to adult role models. Responses were collected through face-to-face interviews. Changes in behavior can be analyzed chronologically, as well as social networks, the contexts of young people’s sexual practices, and the sources of norms and values. The survey also made it possible to assess knowledge about AIDS and its prevention, particularly through the use of condoms.