This chapter on the history of avian influenza viruses starts at the beginning of the 20th century with the description of early fundamental experiments on the characterization and replication of fowl plague virus before it was recognized as an influenza A virus. Virus replication is explained using molecular approaches, and the importance of the cleavability of the hemagglutinin for starting the infection, for pathogenicity, organ tropism, and outbreaks is a central theme. The role of host factors for specific modifications of viral components for understanding species specificity is discussed as well as virus genetics leading to the concept of a segmented genome which helps to explain reassortments and creation of pandemic viruses. Studies on the ecology revealed a huge reservoir of avian influenza viruses in waterfowl in evolutionary stasis, from which from time to time an avian virus arises to pass the species barrier to mammals, explaining well the disastrous pandemic of 1918-1919. Possible outbreaks of future pandemics and how they might be dealt with are also discussed.