Obesity is an increasingly important health problem for both man and dog. Osteoarthritis (OA) is a significant cause of pain and disability in both species. A link between obesity and OA has been established in man, though the exact mechanism of the relationship remains to be fully elucidated - current research supports both biomechanical and biochemical theories. There is good evidence (class I*) to support weight loss as an effective treatment for human knee OA. In the dog, the relationship is just beginning to be investigated. The results of one study in dogs (class IV evidence*) suggest that preventing the development of overweightness and obesity reduces the prevalence of hip dysplasia and OA of the hip and other joints. Three other studies (class III and IV evidence*) support weight loss as an effective treatment for OA in affected overweight and obese dogs. Further research could yield greater understanding of the pathophysiology of this relationship, perhaps identifying novel therapeutic targets. Confirmation and better understanding of the positive effect of treating and preventing obesity on symptoms and prevalence of OA is likely to be valuable in the campaign against canine obesity.