Previous research on friendship selection has failed to focus on how the selection process changes with age or in differently organized environments. This review of the literature takes a life-course perspective on the selection of friends, charting research results on three aspects of the selection process: (1) the number of friends and their proximity; (2) the visible features of friends such as age, sex, and race; and (3) characteristics of friendships and similarity of friends. Over 250 references were reviewed to examine how selection patterns change with age and under different environmental settings from preschool to postsecondary school settings. The results revealed important developmental patterns. With age and the development of cognitive skills and experiences, older students tend to choose fewer best friends, make choices from wider boundaries, increase cross-sex choices, decrease cross-race choices, move toward mixed-age choices, reciprocate and stabilize friendships, and choose more similar friends. There were also important environmental effects on the choice of friends. For example, elementary, junior high, and high schools may be organized to reward or to ignore or punish cross-race or mixed-age choices of friends; or to emphasize differences or similarities among students. These and other environmental conditions affect selection in ways that revise expected patterns of choosing friends. (Author/JAC)