The development of biological monitoring for persons exposed to organic solvents is described. The advantages of this supervision strategy, as well as the reservation against it, are discussed. With the target organs of organic solvents, matrices and parameters for biological monitoring taken into consideration, the following recommendations can be given: the determination of solvent level in blood or alveolar air as a substitute is obligatory for the biological monitoring of organic solvents (the toxic stress of the target organ--the brain--is reflected this way); metabolites have to be determined also when they or their predecessors impair other organs. The discussion of chemical analysis, interference and influencing factors, as well as biological limit values, leads to suggestions of how to interpret the results of biological monitoring. Beyond the estimation of the health risk of the individual worker, the results also have to be interpreted on a group basis. Only in this way can all information on biological monitoring be used to the advantage of prevention. In summary, today biological monitoring is possible for most of the organic solvents. Estimation of health risk in the case of simultaneous exposure to several solvents is still not possible. Parameters measuring toxic strain must be established. The various social groups have to be convinced of the advantages of biological monitoring.