This study investigated selection, deselection, and influence processes of happiness in adolescent friendship networks. Longitudinal data on friendship networks and happiness of 426 adolescents (M = 15.78, SD = 0.65) were analyzed, using stochastic actor-based models. Although happiness similarity did not predict friendship formation (selection), happiness dissimilarity predicted friendship dissolution (deselection). In addition, adolescents influenced each other's happiness over time (influence). Our findings indicated that happiness influence can involve either increases or decreases in adolescent happiness, depending on the average level of happiness of their friends. The results highlighted that, in order to understand the development of adolescents' happiness, it is important to examine the happiness of their friends.