Starting from a political socialization perspective, this study examined the development of political participation during adolescence and early adulthood. We explore the effect of parents, peers, school media, and voluntary associations on political participation. Self-reported data were collected from 3,025 Belgian adolescents at three points in time: at age 16, age 18, and age 21. Latent growth curve modeling was conducted to analyze the effect of the five agents of socialization on the initial level and development of political participation. As hypothesized, we find that all political socialization agents influence the initial level and development of political participation over time. Peers and voluntary associations have the largest influence on the initial level of political participation and on its development. Parents and school would appear to be of less importance. While watching more television has a negative effect, more news consumption and internet use leads to more political participation.