Soil microbial biomass is an important ecological parameter because it acts as a source/sink in nutrient cycling processes and regulateds many organic matter transformations. Appropriate methodologies must be developed to measure the microbial pools in natural and man-managed systems (agroecosystems). A group of techniques has been developed to measure microbial size/activity. The methods summarised include enumeration techniques, respiration, CHCl3-fumigation techniques, substrate-induced respiration, ninhydrinreactive N and the adenosine triphosphate analysis. The CHCl3-fumigation incubation and extraction methods are also useful to recover labelled tracers (e.g., 14C and 15N) from the microbial biomass. Each of these methods has particular advantages and limitations that depend on the nature of the research questions and objectives.