(HPG) axis function fundamentally affects the physiology of eating. We review sex differences in the physiological and pathophysiological controls of amounts eaten in rats, mice, monkeys, and humans. These controls result from interactions among genetic effects, organizational effects of reproductive hormones (i.e., permanent early developmental effects), and activational effects of these hormones (i.e., effects dependent on hormone levels). Male-female sex differences in the physiology of eating involve both organizational and activational effects of androgens and estrogens. An activational effect of estrogens decreases eating 1) during the periovulatory period of the ovarian cycle in rats, mice, monkeys, and women and 2) tonically between puberty and reproductive senescence or ovariectomy in rats and monkeys, sometimes in mice, and possibly in women. Estrogens acting on estrogen receptor-␣ (ER␣) in the caudal medial nucleus of the solitary tract appear to mediate these effects in rats. Androgens, prolactin, and other reproductive hormones also affect eating in rats. Sex differences in eating are mediated by alterations in orosensory capacity and hedonics, gastric mechanoreception, ghrelin, CCK, glucagon-like peptide-1 (GLP-1), glucagon, insulin, amylin, apolipoprotein A-IV, fatty-acid oxidation, and leptin. The control of eating by central neurochemical signaling via serotonin, MSH, neuropeptide Y, Agouti-related peptide (AgRP), melanin-concentrating hormone, and dopamine is modulated by HPG function. Finally, sex differences in the physiology of eating may contribute to human obesity, anorexia nervosa, and binge eating. The variety and physiological importance of what has been learned so far warrant intensifying basic, translational, and clinical research on sex differences in eating.neuroendocrinology; estrogens; testosterone; eating disorders; obesity SEX DIFFERENCES ARE PERVASIVE in physiology and medicine (51,64,73,109,110,466,797,826). The controls of eating and energy homeostasis are no exceptions. It was observed approximately 100 years ago that removal of the ovaries leads to marked accretion of adipose tissue in rats (697), that daily food intake expressed as kilocalories per gram body weight differs between male and female rats (778), and that food intake varies regularly through the ovarian cycle in intact female rats (674, 779). Sex differences in eating have been the subject of physiological research ever since. The clinical relevance of this work is increasingly evident. In the United States, women are approximately threefold more vulnerable than men to psychiatric eating disorders (346,351) and approximately twofold more vulnerable to severe and morbid obesity (BMI Ն 35 and 40 kg/m 2 , respectively, mass/height 2 ) (226). Women also appear to suffer more from these disorders in terms of physical and psychological functioning and quality of life (24,84,273,292,465,531,762). The increased obesity burden suffered by women is reflected in the fact that Ͼ80% of bariatric surgery patients in the U...