SUMMARY Soil microorganisms play a key role in soil biogeochem-ical cycling processes, in sustaining natural soil ecosystems and in agricultural production. Microbial diversity is an index of the community that takes into account both species richness and the relative abundance of species. This paper discusses concepts of microbial diversity from various points of view and extends the concepts to include genetic diversity. Advantages and disadvantages of various research methods (i.e., methods involving culturable microorganisms, biochemical markers and indirect methods, microscopy and other direct methods, and nucleic acid methods) to study microbial diversity are listed to help match research goals with the appropriate research method. Recent progress has yielded a better understanding of microbial diversity in selected environments and these results are reviewed with emphasis on the plant root rhizosphere and some typical agroecosystems. The study of microbial diversity and how it relates to sustainable agriculture and environmental protection is an emerging focus area that has great potential. We believe that advances will continue to occur as we apply new and reliable research methods both in the laboratory and in the field.