The causes of hepatitis in dogs are mostly unknown. Known causes of canine hepatitis are infectious (CAV-1), toxic (e.g. aflatoxin), and metabolic (copper accumulation). In order to understand the unknown causes, research in this field is necessary. Despite the marked progress in the knowledge on viral causes for human hepatitis, the involvement of infectious agents in the pathogenesis of hepatitis in the dog is still largely unknown. It is, like in human hepatitis, very likely that more than one causative infectious agent may cause hepatitis in the dog. This review presents the various forms of hepatitis in the dog, the known infectious and non-infectious causes of canine hepatitis, the infectious causes of hepatitis in man and other animals, and finally our recent infection and molecular studies to investigate possible infectious causes of canine hepatitis.