Conventional, non-conventional and emerging techniques to detect and monitor genetically engineered microorganisms (GEMs) or rDNA sequences in the environment are described. Where appropriate, advantages and disadvantages of each technology are discussed. One basic requirement of a monitoring technique is its ability to detect low levels of released rDNA sequences in an environment. There is also a need to test and apply future or emerging techniques for monitoring the GEMs, such as the use of phage attachment and sensitivity and development of new selective enrichment techniques. Also, the use of differential genotypic markers, for example, the insertion of the beta-galactosidase genes or lux genes, into the organisms of interest, appears to be a very selective and specific technique. While further developments and refinements are necessary, relatively efficient routine technology is on the horizon.