The fate and survival of genetically engineered microbes is dependent on the survival, establishment, and growth of the microbial host, as well as on the maintenance, replication, and segregation of the recombinant plasmids within the bacterial host population. The interactions of the host and vector with each other and the environment influence the ability of the novel microbe to survive, grow, and possibly transfer its genetic information. Several of these host-vector interactions, i.e., plasmid maintenance, bacterial host fitness, physiological changes in the bacterial host, and gene transfer, have been studied using genetically engineered plasmids. These studies indicate that certain biotic factors, such as competition between the engineered microbe and the indigenous microbiota, generation time, plasmid size and copy number, and abiotic factors, such as clay minerals, nutrients, and moisture, influence the survival of any genetic transfer by engineered microbes in the environment.