Technical Abstract: Influenza viruses are clinically and economically important agents of disease in people, horses, pigs, marine mammals and poultry. Human influenza results from infection with influenza A, B or C viruses and a wide variety of domestic and free-ranging wild animal species can be infected with influenza A viruses. Aquatic birds are the natural hosts of influenza A viruses and represent a vast, global reservoir of influenza genes. Because pandemic influenza is fundamentally a zoonotic disease involving interspecies transmission of viruses from animals, this chapter jointly reviews the epidemiology, ecology and evolution of influenza viruses among humans, birds, and pigs. The epidemiologic consequences of genetic reassortment and adaptation of influenza viruses in these species and interspecies transmission are discussed. The epidemiology of interpandemic human influenza is presented with an emphasis on events of the past decade.