Children born shortly before the cutoff date for age grouping in youth sport programs suffer from being promoted to higher age groups earlier than their laterborn peers. Skewed birthdate distributions among participants in youth sport and professional sport leagues have been interpreted as the result of this disadvantage. A growing body of research shows that this Relative Age Effect in sport is a worldwide phenomenon and that it exists in many, but not all, competitive sports. Both physical and psychological mechanisms that may be responsible for the effect are identified. Negative consequences on personal development and possible remedies to the problem are discussed. Finally, desirable and necessary directions for future research are formulated. © 2001 Academic Press Most public school systems specify cutoff dates for entrance into kindergarten or first grade. Although a primary purpose of this procedure is to avoid large age differences, children born shortly after the cutoff date are still up to a full year older than late-born children in their respective age group. This difference of age between individuals in the same age group is referred to as relative age, and its consequence is known as the Relative Age Effect (RAE). A maximum relative age difference of almost 1 year, which is typical of the public school system in most countries, is associated with significant differences in children's cognitive development (Bisanz, Morrison, & Dunn, 1995;Morrison, Smith, & Dow-Ehrensberger, 1995). It isWe are grateful to Günther Bäumler and Roy Hay for stimulating discussions and comments on the RAE phenomenon. Two anonymous reviewers also helped to improve the manuscript.