Over the past decade we have witnessed a major increase in the scale of scientific activity devoted to the study of energy balance and obesity. This explosion of interest has, to a large extent, been driven by the identification of genes responsible for murine obesity syndromes, and the novel physiological pathways revealed by those genetic discoveries. Others and we have also recently identified several single gene defects causing severe human obesity. Many of these defects have been in molecules identical or similar to those identified as a cause of obesity in rodents. I will review the human monogenic obesity syndromes that have been characterised to date and discuss how far such observations support the physiological role of these molecules in the regulation of human body weight and neuroendocrine function.