No one wants to overeat. And certainly no one wants to overeat for years, become overweight, and end up with a high risk of diabetes or heart disease—yet two thirds of Americans do precisely that. Why does our behavior betray our best intentions to eat healthy foods in moderation? The reason is that our appetites and food choices are led astray by ancient, instinctive brain circuits that play by the rules of a survival game that no longer exists. These circuits don’t care about how you look in a bathing suit next summer. Join Stephan J. Guyenet for an exploration of some of these circuits and how they conspire with our modern food environment to expand our waistlines.