Although it is far from certain that HPAIV H5N1 will transform into the next pandemic human influenza virus, the possibility of this to occur and the worries of politicians and the public of catastrophic consequences of an overdue influenza pandemic resulted in enormous efforts to improve diagnosis, to better understand the epidemiology of avian influenza, to assess the molecular basis for influenza virus virulence and host specificity, and to design improved vaccines for animals and humans alike. This drastic increase in research has resulted in a significant expansion of our knowledge of avian influenza viruses and the role they play in animals and humans. This issue of the OIE Review presents a comprehensive overview of our present understanding of avian influenza. Foremost experts in influenza have agreed to share their opinion and latest results to provide a state-of-the-art summary on the current knowledge.