Influenza is known as a human disease, but many influenza viruses can be found in several other animal species. In some it induces a disease similar to man's infection with a major respiratory tropism. In others the disease is quite different, like the systemic infection of fowl plague. In others, it can be totally asymptomatic. The main species concerned are birds, horse, pig, felines, canines, marine mammals and mustelids. The mechanisms of interspecies transmission of the virus are now better understood and they condition influenza epidemiology. Lessons from the study of past pandemics and other manifestations of influenza during 20th century help to design the tools and procedures for managing an eventual new pandemic.