A number of debates centring on the so-called 'obesity epidemic' have sprung up in the last 10 years. Although debate is the normal state of affairs in any scientific endeavour, my focus here is on two broad camps we might call 'alarmists' and 'sceptics'. While alarmists have characterised the 'obesity epidemic' as a looming global health catastrophe, sceptics have argued that the consequences of rising obesity levels have either been greatly exaggerated or are unclear. In focusing on obesity, my intention is not to prove the case of one camp or the other but rather to construct a kind of anatomy of the obesity controversy. In this essay, I want to move beyond the idea of there being two camps in debates about obesity and provide a more complex account of the different groups that make up both sides. My interest in this research is to explore the idea that belief matters more than truth. In the context of debates about obesity, nothing could be more irrelevant than the 'truth' of fatness. The energising principle will be what people, particularly but not only politicians and journalists, can be made to believe.