Pulsed-field gel electrophoresis was developed for separating and analyzing of long DNA fragments in alternating electric field. In homogenous electric field, fragments longer than 50 kb run as a broad, unresolved band with high mobility. PFGE separated the DNA by periodicaly changing the direction of electric field. DNA molecules are moving "zig-zag" through the gel and they can be better separated. Fragments of several megabases can be resolved using this method. PFGE can be used for genome mapping of microorganisms as well as higher organisms. In microbiology, PFGE is a standard method for typization of bacteria. Comparison of electrophoresis profiles after digestion od DNA from bacterial isolates with restriction endonuclease is a very useful epidemiologists tool. Genetically identical organisms have the same PFGE profiles, different strains have different profiles. Related strains have also similar electrophoretic profiles. This enables to determine if the outbreaks are caused by the same strain of microorganism, to locate the source of outbreak and to monitor the spread of the microorganism. The most followed-up are nosocomial and the food-borne pathogens. PFGE can be also used for monitoring genetic evolution of the microorganism and the most prevalent types which circulate in population. This can be very useful for preparation of vaccines.