By using the techniques of molecular biology, such as multilocus enzyme electrophoresis and multilocus sequence typing, it has been possible to identify the clones of meningococci that have been responsible for major epidemics in the world and to elucidate the routes of spread of these bacteria. Although meningococci can rearrange very rapidly their genome through the process of transformation, some clones or some groups of closely related clones have been stable and have been associated with increases of incidence of disease during decades. A clone with an epidemic potential can be disseminated globally within a few years, but the reasons for the development of an epidemic in a particular population are still not fully understood.