Why do friends matter to us? How do friendships affect children’s and adolescents’ social and emotional development? Are friendships only positive influences, or can friends also lead us astray? Can friends buffer vulnerable youth against challenges in the peer world? These are the kinds of questions I ask in my research, and they all address the goal of understanding the developmental significance of friendship—how friendships contribute to our adjustment and well-being. My work is in the area of developmental psychopathology at the intersection of developmental and clinical psychology. In early studies, I considered the long-term significance of having a close friend and the friendships of specific groups of children, such as highly aggressive children and children with ADHD. More recently, I have worked with colleagues to investigate connections among friendships and peer victimization, including the friendship quality of children who are victimized and the role of a close friend in buffering against peer victimization. In addition, my students and I have investigated friendship in early adulthood and considered links between friendship and happiness.