Since the 1930s, traditional methods of strain identification based on serotyping and phage typing have been the foundation of salmonella epidemiology. Although the incidence of diseases such as typhoid and paratyphoid has decreased in recent years, food-poisoning caused by non-typhoidal salmonella strains has now reached epidemic proportions in many countries, despite improvements in sanitation and hygiene. Precise strain identification is an essential prerequisite for epidemiological investigations aimed at combating the spread of these strains and eradicating the sources of infection. Modern methods of genotypic typing, particularly those based on physical characterization of the plasmid content of the organism have already proved invaluable for the identification and differentiation of strains in many outbreaks. These plasmid typing methods are now increasingly used with serotyping and phage typing for many epidemiological investigations. Other methods of genotypic typing, particularly those based on recognition of small differences in chromosome structure, are not yet practical for the examination of large numbers of strains. Nevertheless, improvements in small-scale methods for chromosomal DNA extraction coupled with the increasing use of non-isotopic labels for identification of restriction fragment length polymorphisms may provide a new dimension to Salmonella epidemiology.