AbstractInfluenza A viruses cause recurrent epidemics and global pandemics. One of the unique features of influenza virus is the ability to overcome interspecies barrier. Reassortment of viral genes and the accumulation of mutations contribute to the emergence of new influenza virus variants. The replication of influenza A virus in a specific host depends on many factors e.g. activity of viral proteins, host response system and environmental conditions. In this review the role of viral proteins as a condition for crossing the species barriers is discussed.