Biological monitoring of chemical exposure in the workplace and in the environment has become increasingly important in assessing health risk. The analysis must be carried out under a quality assurance scheme to guarantee that the results obtained in biological monitoring are comparable with the threshold limit values and results from other laboratories. Since 1982, the German Society for Occupational and Environmental Medicine has offered 28 intercomparison programs. These programs cover 96 analytes in urine, blood, and plasma for 47 substances and cover most parameters that are relevant in occupational and environmental medicine. The data obtained in these programs provide a good overview of the current quality of the determination of analytes that are assessed in occupational and environmental toxicological laboratories. For the analyses of inorganic substances in blood and urine, the tolerable variation ranges from 7.5% to 43.5%. For organic substances in urine, the tolerable variation ranges from 12% to 48%. For organic substances in urine, the tolerable variation ranges from 12% to 48%. The highest variations (36% to 60%) were found for the analysis of organochlorine compounds in plasma. The tolerable variations for determining solvents in blood by head space gas chromatography range from 26% to 57%. The overall average success rates for the participants of the external quality programs range from 65% to 75%.