This paper reviews the current state of knowledge as regards the antigenic and biochemical heterogeneity of influenza viruses. It deals particularly with the influenza A virus, which has the unique features of changing antigenic structure and various factors related to virulence. These properties cause widespread epidemics at unpredictable intervals and give rise to concern among vaccine manufacturers and epidemiologists in the health services, but also provide biochemists and geneticists with a fascinating model for basic biological research. The present-day epidemiological situation with two influenza A subtypes circulating simultaneously is described.