This chapter reviews and integrates research on the importance of friendships to well-being, with a focuses on adults. It begins with the question "What is a friend?" This question has been answered in the popular media, by laypeople, and by social scientists. The chapter presents these answers and point out common threads. It also reviews these areas of research, as well as research on variations in conceptions of friendship as a function of individual differences and culture. The chapter discusses whether social media has changed the definition of this kind of relationship. Macro-level perspectives on friendship also focus on various aspects of the friendship network. In sum, in macro-level approaches, friends are viewed as part of a larger network of social ties. Relationship scientists have been interested not only in defining friendship, but examine how definitions vary as a function of sexuality and gender, including gender of participants, gender composition of the friendship and sexual orientation or identity.