A review is given of real and potential risks connected with the use of conventional and genetically engineered live and dead vaccines. Special attention is given to live carrier vaccines expressing one or more heterologous genes of other microorganisms. Because most carrier vaccines are still in an experimental phase, there is only limited experience with the risks of carrier vaccines. There are three potential risks of live carrier vaccines which will be discussed: 1. Changes in cell, tissue, of host tropism, and virulence of the carrier through the incorporation of foreign genes. 2. Exchange of genetic information with other vaccine or wild-type strains of the carrier organism. 3. Spread in the environment. Only limited experimental data are available on changes in biological behaviour of microorganisms through the incorporation of foreign genes. For example, there are indications that vaccinia virus carrying the attachment protein G of respiratory syncytial virus (RSV) replicates better in lungs of mice than vaccinia virus carrying other genes of RSV. Poxviruses carry genes that probably determine their replication in different hosts. Exchange of such host tropism genes might alter their host spectrum. Recombination between herpesvirus vaccine or wild-type strains may lead to the appearance of virulent strains with of without heterologous genes. Before carrier vaccines are applied, these risks must be thoroughly evaluated case-by-case. Potential methods for the design of safe carrier vaccines are discussed.