In this introductory article of this special issue, we introduce Project STARS (Studies on Trajectories of Adolescent Relationships and Sexuality), a longitudinal research program on the development of romantic relationships and sexuality of adolescents in the Netherlands, which was conducted between 2010 and 2015, and was funded by the Netherlands Organization for Scientific Research (NWO) and the Fund for Scientific Research of Sexuality (FWOS), as part of the NWO program Youth and Family. The research program consisted of four interrelated sub-projects. In each of these sub-projects, longitudinal data from 1,297 adolescents were used. In addition, several quantitative and qualitative in-depth studies were conducted within each of the sub-projects. In this article, the background, the design and the innovative aspects of Project STARS are discussed.