Biological limit values (BLVs) were discussed. The distinction between BLVs and environmental limit values (ELVs) was described. ELVs such as threshold limit values or recommended exposure limits were derived from the relationship between the airborne concentration and an adverse health effect of an industrial chemical in laboratory animals or humans, whereas BLVs were based on the relationship between a measure of internal dose and the environmental exposure or a health effect. ELVs depended on the validity of airborne sampling and analysis methods and did not accurately access exposure by routes other than inhalation. BLVs provided information useful for evaluating a worker's response and measuring his overall exposure and can be regarded as reference guides for assessing the results of biological monitoring data. BLVs depend on many more factors, such as metabolism and pharmacokinetics of the chemical and body build, workload, and lifestyle of the worker, than ELVs. Lauwery's triangle, a schematic for describing the interrelationships between environmental exposure, internal dose, and health effects was described. The biological exposure index (BEI) of the American Conference of Industrial Hygienists was reviewed. The biological tolerance value (BAT) of the Federal Republic of Germany was discussed. Industrial chemicals for which international BEIs exist were summarized. Sources of suggested BEI values were described. The German approach for BATs for established carcinogens was described. The approach stipulates that BATs are not assigned for known carcinogens because no biological standard that can be regarded as safe can be specified.