Since the pioneering studies of Edward Jenner on using live virus vaccines during the early 1800s (Baxby 1981), a considerable amount of information has been obtained on the biology and replication of poxviruses. Vaccinia virus, the agent used to vaccinate humans against smallpox and eradicate this disease, has been widely studied in the laboratory due to their large size and ease of growth in tissue culture. With advances in molecular biology and genetic engineering techniques during the past two decades, a large amount of information on the genomic organization and gene expression in vaccinia virus has accumulated. With this information came the ability to insert and express foreign genes under the control of vaccinial promoters (Panicali and Paoletti 1982; Mackett, Smith and Moss 1982). What follows in this section will be a brief description of the basic virology and molecular biology of poxviruses, with emphasis on those aspects important in using poxviruses as expression vectors. (Cited references are meant to be illustrative but not comprehensive. For more indepth coverage, the reader is directed to the following reviews: Fenner, Wittek and Dumbell 1989; Moss 1990a; Buller and Palumbo 1991; Moss 1992).