The classical viral vaccine approaches using inactivated virus or live-attenuated virus have not been successful for some viruses, such as human immunodeficiency virus or herpes simplex virus. Therefore, new types of vaccines are needed to combat these infections. Replication-defective mutant viruses are defective for one or more functions that are essential for viral genome replication or synthesis and assembly of viral particles. These viruses are propagated in complementing cell lines expressing the missing gene product; however, in normal cells, they express viral gene products but do not replicate to form progeny virions. As vaccines, these mutant viruses have advantages of both classical types of viral vaccines in being as safe as inactivated virus but expressing viral antigens inside infected cells so that MHC class I and class II presentation can occur efficiently. Replication-defective viruses have served both as vaccines for the virus itself and as a vector for the expression of heterologous antigens. The potential advantages and disadvantages of these vaccines are discussed as well as contrasting them with single-cycle mutant virus vaccines and replicon/amplicon versions of vaccines. Replication-defective viruses have also served as important probes of the host immune response in helping to define the importance of the first round of infected cells in the host immune response, the mechanisms of activation of innate immune response, and the role of the complement pathway in humoral immune responses to viruses.