Adequate nutrition is essential for normal reproductive function in man. Menarche occurs at a critical level of 'fatness' and it appears that the hypothalamic events leading to pubertal development and the achievement of reproductive competence may be triggered by metabolic/endocrinological changes due to an increase in fat. The attainment and maintenance of ovulatory cycles demands a minimum degree of body fat (about 22%). Undernutrition results in weight loss and a reduction in fat tissue. This alteration in body composition in turn precipitates the hypothalamic changes that cause impaired gonadotrophin secretion, inappropriate ovarian stimulation and menstrual abnormalities--usually primary or secondary amenorrhoea. Refeeding with restoration of fat tissue will usually result in recovery of reproductive function. There are many causes of undernutrition and the main ones have been discussed. In developed countries the dietary restriction is usually self-imposed--either by rigid control of intake or by the high energy demands of intensive exercise. In contrast to this 'starvation amidst affluence', in developing countries malnutrition is usually the result of inadequate food supplies and poverty. While infertility is often the consequence of undernutrition, many women with suboptimal nutritional states do succeed in becoming pregnant. Malnutrition prejudices pregnancy outcome and results in increased maternal and fetal morbidity and mortality as well as long-term developmental sequelae in the infant. Dietary supplementation is helpful but cannot always overcome the insult caused by the periconceptional undernutrition. The cessation of reproductive function in the undernourished woman represents an adaptive phenomenon, since pregnancy would be prejudicial both to her and the fetus. In view of this, treatment should always aim at refeeding and not at induction of ovulation. Malnourished communities require dietary support and they present the problem of provision of adequate resources. So-called ethnic and culture differences in pubertal development and reproductive performances are in fact often the result of discrepancies in resources and nutrition. Given equal nutritional opportunities these variations may well disappear within a few decades.