Live vaccines are composed of viral or bacterial strains which are deprived of their pathogenicity but can still replicate in the organism. These vaccines can provoke a non pathogenic infection in vaccinated subjects, both inducing an antibody and a cellular immune response. Live attenuated vaccines such as measles, rubella, mumps or yellow fever vaccines have been widely used with proven efficacy. The oral poliomyelitis vaccine is composed of three attenuated strains prepared from the three wild virus types and is efficacious but mutations of the virus (in particular for type 3) can induce polio-associated paralysis by giving it back its original neurovirulence (number of cases 2:1,000,000). Other vaccines prepared from empirically attenuated strains such as varicella, dengue or cytomegalovirus strains are currently developed. The preparation of vaccine strains using gene deletion or attenuation directed mutagenesis makes it possible to develop highly genetically stable vaccines, in particular against orally transmitted bacterial diseases (e.g. typhoid fever, shigella, cholera). However, the future certainly belongs to combined live vaccines where genes encoding vaccine antigens are inserted into non pathogenic viruses or bacteria administered to man and are then expressed in the organism. The most promising model seems to be the one using poxviruses.