Influenza A viruses are zoonotic pathogens that continuously circulate and change in several animal hosts, including birds, pigs, horses and humans.The emergence of novel virus strains that are capable of causing human epidemics or pandemics is a serious public health concern, and continues to cause problems all over the world.I will discuss the value of surveillance and characterization of naturally occurring influenza viruses, and the impact that reverse genetics techniques developed in our laboratory have had on our understanding of the host tropism and virulence of influenza viruses.I will also discuss the use of these techniques in promising attempts to improve the existing influenza virus vaccines.