In this article, we review what is known about the formation of friendships and friends' influence in adolescence with a focus on quantitative and social network studies. First, we describe how friendships are defined and measured. Second, we provide an overview of mechanisms of friendship formation. In the remainder of the article, we summarize research findings on the formation of friendships, the development and stability of friendships, how socio-cultural context affects friendships, and the influence of friendships.
