The determinants of the timing of menarche are many, and it is difficult to isolate a single factor which may delay or accelerate this maturational event. Menarche, on average, occurs later in athletes, including ballet dancers, than in the general population. With few exceptions, the data are consistent across several countries. Swimmers are an exception to the tendency towards later menarche in athletes. In all samples of swimmers surveyed but one, menarche tends to approximate the average for the general population. The menarcheal data are consistent with other indices of biological maturity status, i.e., skeletal age and secondary sex character development. The data dealing with the effects of intensive physical training on menarche are reviewed and given the number of factors involved, it is difficult to implicate training as a factor which specifically delays menarche. A two-part hypothesis for the apparent maturity delay in athletes is thus suggested. The hypothesis combines biological selective factors (i.e., physique and skill) and social factors. Family size and composition, however, must receive more consideration. Family size influences the timing of menarche and such a trend is apparent in data for several groups of athletes.