It has been hypothesized that hypoxia reduces fertility, but comparative studies of high and low altitude populations have been unable to verify or refute this proposal because it is difficult to control for the behavioral and sociocultural factors that may also either underlie fertility differentials or compensate for physiological changes caused by varying partial pressure of oxygen. Taken collectively, estimates of fertility in populations exposed to chronic hypoxia range widely and do not suggest any reproductive patterns specific to high altitude. Here we review the available data from the Andes and the Himalaya on the proximate determinants of fertility, that set of factors through which any and all environmental, behavioral, and sociocultural factors must act to influence fertility levels. Although hypoxia could potentially affect some of these factors, there is no unequivocal evidence that this occurs in human populations indigenous to high altitude. At this time, it appears that local variations in infant feeding beliefs and practices, often coupled with prolonged breast-feeding, play a major role in determining fertility variation in Andean populations. In the Himalaya, large numbers of adults are not in sanctioned sexual unions as a consequence of polyandrous marriage practices and religious celibacy. The absence of a clear negative effect of hypoxia on fertility in populations indigenous to high altitude, even though migrants report reproductive difficulties, argues that these populations have adapted to the conditions at high altitude. The experimental and clinical evidence presented in this issue suggests proximate mechanisms by which such adaptation, shaped by natural selection and developmental processes, is possible.