Similarity is central to friendship. The origins of homophily (love of the same) between friends are varied, but they all serve the same purpose: fostering compatibility and maintaining the rewards of affiliation. Interpersonal attraction is grounded on similarity. Similar individuals have much in common and find it easy to get along. Friends are selected on the basis of resemblances and friends influence one another to become more similar over time. Friends also resist changes that promote dissimilarity because differences increase the risk that the friendship will dissolve. Similarity is an essential feature of friendship at all stages of life, but it assumes special significance during adolescence when changes in autonomy coincide with changes in the social world to raise the profile of friends.