question: The new technologies of genomics, proteomics, and molecular immunology have led to an impressive increase in our knowledge of all aspects of virology, providing insights that can guide new vaccine concepts. When developing a new vaccine, the choice of approach is made very much on a case-by-case basis, and for a given virus is driven by knowledge its pathogenesis, serotype diversity, antigenic variation, immune evasion mechanisms, latency and route of transmission. High importance is given to what type of immunity arises as a result of natural infection and whether the pathogen can cause persistent and/or repeated infections in a single host. This article discusses the medical need for new human viral vaccines and focuses on the strategies available for their development and some of the challenges posed by the more difficult targets.