In 1996, an estimated 52.9 million dogs were living in 31.6 percent of all households in the United States (1). Several epidemiologic studies have been undertaken to identify risk factors for cancer in pet dogs. This presentation reviews the literature on canine cancer epidemiology with a view towards exploring how these studies inform us about cancer causation in dogs and in humans. It focuses on observational studies of dogs living in their usual environments, and considers experimental studies in laboratory settings only to the extent that such studies shed light on the observational studies. It begins with a discussion of some methodological issues that make epidemiologic studies of cancer in dogs somewhat different from those in humans.