Obesity is a big problem. In recent years, the increasing incidence of obesity in companion animal species, particularly dogs, cats and horses, has attracted headlines. Near universally, poor owner management of diet and exercise are blamed for this increase in the press. However, there is a weight of evidence that obesity is perhaps best viewed as a complex homeostatic mechanism gone awry, influenced by genetics. In this lecture, I will review the role of genetics in influencing obesity susceptibility in dogs and other species and touch on what genetic studies have taught us about obesity biology.