Avian influenza (AI) is a complex infection of birds, of which the ecology and epidemiology have undergone substantial changes over the last decade. Avian influenza viruses infecting poultry can be divided into two groups. The very virulent viruses cause highly pathogenic avian influenza (HPAI), with flock mortality as high as 100%. These viruses have been restricted to subtypes H5 and H7, although not all H5 and H7 viruses cause HPAI. All other viruses cause a milder, primarily respiratory, disease (low pathogenic avian influenza, LPAI), unless exacerbated by other infections or environmental conditions. Until recently, HPAI viruses were rarely isolated from wild birds, but for LPAI viruses extremely high isolation rates have been recorded in surveillance studies, particularly in feral waterfowl. In recent years, there have been costly outbreaks of HPAI in poultry in Italy, the Netherlands and Canada and in each of these countries millions of birds were slaughtered to bring the outbreaks under control. However, these outbreaks tend to have been overshadowed by the H5N1 HPAI virus, initially isolated in China, that has now spread in poultry and/or wild birds throughout Asia and into Europe and Africa, resulting in the death or culling of hundreds of millions of poultry and posing a significant zoonosis threat. Since the 1990s, AI infections due to two subtypes, LPAI H9N2 and HPAI H5N1,have been widespread in poultry across large areas of the world, resulting in a modified eco-epidemiology and a zoonotic potential. An extraordinary effort is required to manage these epidemics from both the human and animal health perspectives.