There may be many social psychological variables that influence or are influenced by children's behavior in organized sports. The major variable discussed in this paper is the child's motivation to participate. One cognitive theory--the attribution theory-- offers insights into the child's view of his motivation, and the effects upon this motivation of rewards, authority figures, and winning and losing. In particular, the "overjustification" hypothesis of attribution theory may have implications for the physical education and athletic programs of children. Judging by their entrance into the gymnasium or onto sport's fields and playing areas, most children come to these activities with high intrinsic motivation. It is possible that through our grading and awards system we decrease the strength of this intrinsic motivation while we strengthen the need for external rewards, extrinsic motivation. It is also possible that our entire athletic system is designed to cause a shift in the intrinsic motivation to play to the extrinsic motive of playing for the reward. Many people have suggested external rewards for all who participate. If the overjustification hypothesis is correct, this might be the worst possible thing to do. The point is not the size of the reward, but simply that when a reward is offered it results in a logical reason to which the child can attribute his motive for playing, an extrinsic rather than intrinsic motivation. Other theories and principles discussed here that shed additional light on the subject of children's motivation to participate in sports activities are: self-perception theory; the discounting principle; the additive principle; and activation-arousal theory. (MM)