This chapter describes the ways in which online affinity networks motivate learning and support interest development. It builds on the model of "connected learning" that posits that learning is most resilient and meaningful when it is tied to social relationships and cultural identities, and spans in-school and out-of-school settings. The analysis draws from ethnographic case studies of youth-centered networks focused on fanfiction, knitting, professional wrestling, anime video remixers, Bollywood dance, YouTube vloggers, and communities surrounding two games, Little Big Planet 2 and StarCraft II. Factors that draw young people to online spaces to pursue their interests are diverse. For some, it is to find a safe space for a stigmatized interest. For others, it is because of an attraction to a narrow niche, leveling up, or technical specialization that is only accessible online. In all cases, however, high functioning online affinity networks are characterized by a strong set of shared values and culture that are the magnet for affiliation. In addition, a set of shared practices provide a focus of activity and engagement.