The recent rapid increase in obesity is undoubtedly due to changes in our lifestyle and in the types of food we eat. Our environment has changed dramatically over the past 50 years, with people taking much less exercise and consuming far more calories than ever before. However, although we are all exposed to these changes, not all of us are obese. Differences in our genetic make-up mean we all respond differently to the same environment. In fact, studies on twins have estimated the ‘heritability’ of body-weight to be anywhere between 40% – 70%. The fact that body weight is a highly heritable trait provides us the opportunity to use genetics as a tool to understand the molecular mechanisms underlying human obesity, thus teaching us about the biology of appetite control.