Promoting the well-being of youngsters is one of the main policy goals of youth policies in many Western countries. Yet a clear and distinct definition of the concept is not available. This paper discusses the measurement of well-being in Belgian youth research and the factors influencing the well-being of adolescents. The paper also describes the time use of Belgian adolescents using data from the 1999 Belgian Time Use Survey, differentiating youngsters by gender, age, educational track and parents' educational background. The 12- to 19-years old, still attending secondary school, are compared with the total population during the school year as well. Finally, we examine what the Belgian Time Use Data reveal on subjective health of young people. Survey-based youth research stresses the growing importance of peers in late adolescence for their well-being. These ideas fit the results of the time use data. The data point to the more physically active leisure of boys and their involvement in games and the so called bedroom culture of girls. As adolescents grow up they are more involved in social leisure, spend less time at home and spend more time with peers. 72.3% of the adolescents report a very good health. These youngsters spend less time on computers, surfing the net and hobbies. They are less alone and spend more time in the company of others. The time use data surprisingly reveal that the total workload of adolescents equals that of the population in general and even exceeds their leisure time. Moreover, adolescents have less free time than the Belgian population at large.