This review weighs the evidence for and against the hypothesis that ovulation is regulated by a critical amount of body fat. The evidence supporting this hypothesis is correlative, and most of it stems from observations made in humans. On balance, the evidence from human studies does not support the hypothesis, however, and the results of animal studies argue strongly against it. In the latter regard, a variety of experimental approaches have been tried in both adult and peripubertal females of several species, and the results almost uniformly show little relationship between fatness and ovulation. There is no doubt that ovulation can be regulated somehow in relation to whole-body energy balance and that fat stores are an important component of energy balance, but there is no reason to accord body fat a direct causal role in regulating ovulation.