Vaccination has been one of the most successful and cost-effective health interventions ever employed. One disease (smallpox) has been eradicated, another (poliomyelitis) should disappear early in the new millennium and a third (measles) should follow shortly after. Conventional vaccines usually depend on one of three development processes, attenuation of virulent organisms (by passage in cell culture and/or experimental animals), killing of virulent organisms (by chemical inactivation) or the purification of immunogenic molecules (either proteins or carbohydrates) from whole organisms. These traditional processes, although serendipitous and poorly understood, have produced effective pharmaceutical products which give excellent protection against diseases such as smallpox, rabies, measles, yellow fever, tetanus and diphtheria. In spite of these successes however, the application of these protocols have failed to produce safe and efficacious vaccines against other infectious diseases which kill or maim tens of millions of people every year. The most important of these are malaria, AIDS, herpes, dengue fever and some forms of viral hepatitis. Consequently, fundamentally new technologies are required to tackle these important infections. One of the most promising has been the development of genetically modified viruses. This process normally involves taking a proven safe and efficacious vaccine virus, such as vaccinia or adenovirus, and modifying its genome to include genes coding for immunogenic proteins from other viruses such as HIV or measles. This review will describe the generation of such novel vaccine vectors and compare their advantages and shortcomings. In addition the literature describing their use as experimental vaccines will also be reviewed.