Viruses are responsible for the onset of various infectious diseases and make a considerable impact on human health. The search for antiviral drugs is significantly assisted by an understanding of the molecular events taking place during viral infections. The research described in this Ph.D. thesis was focussed on enzymes involved in two important stages of the influenza virus life cycle; virus replication, and virus release from the infected host cell. Influenza virus is a significant human pathogen, with high annual infection rates, and three major pandemics recorded in the past 100 years. The continual emergence of new strains of this virus (for example 2009 H1N1 swine influenza virus and H5N1 avian influenza virus) presents an on-going threat to mankind that could have devastating health and socio-economic impact.