Over the past half century, in the United States and other developed nations, children's free play with other children has declined sharply. Over the same period, anxiety, depression, suicide, feelings of helplessness, and narcissism have increased sharply in children, adolescents, and young adults. This article documents these historical changes and contends that the decline in play has contributed to the rise in the psychopathology of young people. Play functions as the major means by which children (1) develop intrinsic interests and competencies; (2) learn how to make decisions, solve problems, exert self-control, and follow rules; (3) learn to regulate their emotions; (4) make friends and learn to get along with others as equals; and (5) experience joy. Through all of these effects, play promotes mental health. (Contains 48 notes and 1 figure.)