Publisher Summary This chapter reviews the studies that have evaluated the survival of bacterial hosts and cloning vectors (e.g., phages) and the transfer of genetic information, by the processes of conjugation, transduction, and transformation, in aquatic and terrestrial environments and on plants. Mention is also made of selected in vivo studies (i.e., in human beings and other animals), as the environmental factors that affect survival and genetic transfer in vivo have similarity to those that affect survival and genetic transfer in situ (i.e., in natural environments). The lack of substantial data on the survival, establishment, and growth of, and on the transfer of genetic information by, genetically engineered bacteria in natural environments has hindered the assessment of the risk to the biosphere of the release, either accidental or planned, of these organisms. Some observations indicate that the acquisition of a plasmid can result in a spectrum of unrelated, unanticipated, and nonpredicted biochemical. Other physiological alterations in the recipient bacteria-suggest that studies designed to evaluate the survival of, and gene transfer by, genetically engineered bacteria in natural habitats be alert for such unanticipated and nonpredicted alterations and that the current wisdom may be erroneous.