The objective of this article is to explain briefly the nature, function and importance of soil microbial biomass in the natural and man-managed ecosystems, in the context of degradation of organic residues; regulation of nutrient cycling and soil organic matter transformations; soil evolution; applications of microbial biomass quantification. Factors (such as soil pH, salinity, inorganic and organic inputs, soil temperature and moisture regimes, soil texture and structure, heavy metals content, management systems and land use) regulating the size, composition and diversity of soil microbial biomass will then be summarised.