Occupationally or otherwise, much of the population is exposed to pesticides. To obtain information on exposure levels, biological monitoring is often the best choice because it provides data that reflects total exposure by all routes. Biological monitoring has been used to evaluate exposure in agricultural and industrial environments, in subjects poisoned by accidental or voluntary contact, in volunteers for pharmacokinetic studies and in the general population. This paper is based on 100 studies published on this topic and reviews exposure indices for the main classes of pesticides. Methods of sampling and conservation of samples are outlined, together with the analytical procedures used. For compounds such as organophosphate and pyrethroid insecticides, ethylenebisdithiocarbamate fungicides, and phenoxyacetic herbicides, the results of studies on the general population and groups of occupationally exposed workers are reported.