Obesity may be the most difficult and elusive public health problem this country has ever encountered. Unlike the classical infectious diseases and plagues that killed millions in the past, it is not caused by deadly viruses or bacteria of a kind amenable to vaccines for prevention, nor are there many promising medical treatments so far. While diabetes, heart disease, and kidney failure can be caused by obesity, it is easier to treat those conditions than one of their causes. I call obesity elusive partly because of the disturbingly low success rate in treating it, but also because it requires changing the patterns, woven deeply into our social fabric, of food and beverage commerce, personal eating habits, and sedentary lifestyles. It also raises the most basic ethical and policy questions: how far can government and business go in trying to change behavior that harms health, what are the limits of market freedom for industry, and how do we look upon our bodies and judge those of others?