Radicalization research is dedicated to analyzing factors related to radicalization processes, which in turn can lead to extremism. One factor is frequently discussed: the role of the Internet. This paper examines the relationship between active and passive online activities, including consumption, networking, and posting, and extremist attitudes in the field of Islamism and right-wing extremism among adolescents. Data from a school survey ( N = 6,715) show that right-wing attitudes are particularly correlated with consumption of political websites, though this effect is weaker among females. However, posting shows only a small effect and networking shows no effect at all. Islamist attitudes, on the other hand, are related to the extent to which one consumes violent Islamist videos, meets other Muslims online, and posts Islamic content.