Influenza viruses belong to the family Orthomyxoviridae. Genus Influenza A viruses are true zoonotic agents with many animal reservoirs, whereas genus Influenza B viruses are generally considered to be a virus of humans. The genome of influenza A viruses consists of eight unique segments of single-stranded RNA of negative polarity; they are typed according to their surface proteins, hemagglutinin (HA) and neuraminidase (NA). HA and NA, the major antigenic determinants of influenza A viruses, are present in 16 and nine serologic subtypes, respectively. Annual epidemics and occasional pandemics of influenza in humans depend on the continued evolution of influenza viruses. Although they have numerous potential host populations, most of our genetic and biologic data are obtained from studies of domestic populations of species such as chickens, turkeys, swine, and horses. Concerning wildlife populations, including wild populations of these domesticated species, much less is known. The purpose of this review is to establish what role wildlife populations play in the continued evolution of influenza viruses. Future work needs to determine what chain of events makes it possible for an influenza virus to be successfully transmitted to, and more importantly within, an alternative host population. Even questions as fundamental as which hosts can transmit viruses to humans remain unanswered so far.