This chapter presents the development and the potential of infectious recombinant vectored virus vaccines. Experimental immunizations with infectious virus vectors carrying one or more heterologous genes for immunogenic proteins have been remarkably successful in experimental animals systems. Such immunizations have enabled the presentation of immunogenic proteins to the host immune system in an authentic way, mimicking closely the way antigens are presented in natural infections. Member viruses of the three families—Poxviridae, Herpesviridae, and Adenoviridae— described in the chapter as having potential for use as infectious vectored virus vaccines, have characteristics that lead to a complex judgment as to the overall promise for the practical use of end products in the field. Feasibility seems clear even though, in most cases, questions of stability of recombinants and the level of expression of inserted gene products have not been fully answered. At present, virus vectors under development have the ability to carry enough heterologous DNA to code for a single or multiple genes for immunogenic proteins. Vector systems allow remarkably faithful transcription and translation from inserted genes, as well as proper posttranslational processing and transport.