Plasmid analysis is a useful method for epidemiologic typing of a variety of organisms. In outbreaks, plasmid profiling is a rapid, convenient way to follow the spread of the epidemic strain and may be more specific than other typing methods. Restriction endonuclease profiling of plasmids has led to an understanding of how the transmission of specific resistance plasmids among many species can contribute to endemic antibiotic resistance. Finally, it is now possible to detect the spread of specific genes among a variety of plasmids. In the future, increased emphasis will be placed on methods for monitoring the spread of specific resistance mechanisms, and research topics will include the spread of specific genes, both on chromosomal and on extrachromosomal DNA; the spread of strains with mutations affecting outer membrane proteins; and changes in the expression of specific chromosomal β-lactamases, Restriction endonuclease digestion of chromosomal DNA, profiling of outer membrane proteins, and monoclonal antibody subtyping will also be used increasingly to monitor the spread of specific strains.