A major concern in the eventual use of genetically engineered microbes (GEMs) to reclaim contaminated soil and water environments is the possible adverse effects of the introduced GEMS on the homeostasis of these and associated environments. The majority of studies with GEMs have been conducted with bacteria, wherein DNA can be transferred in situ by conjugation, transduction, and transformation. Although these phenomea have been demonstrated in a wide spectrum of gram-positive and gram-negative bacteria in the laboratory, there is sparse information on their occurrence in soil, water, and other natural habitats. Much of the literature on the occurrence of these phenomea in situ has been recently reviewed and only some illustrative examples of studies on the survival of, and genetic transfer by, GEMs in soil will be presented. Details on the methodlogies used in these studies can be found in the referenced publications.