The assessment of health risks due to occupational and environmental exposures has become, over the course of the years, increasingly important. Exposure and biochemical effects are estimated by biological monitoring, through the systematic collection of specimens from potentially exposed humans. In the field of biological monitoring, quality assurance (QA) concerns all those actions necessary to provide confidence that the results of laboratory tests will satisfy defined standards for quality. QA includes all steps which have to be taken to ensure that laboratory results are reliable, i.e., the adoption of good, scientifically sound practices for laboratory operation, including-apart from all the analytical steps-selection, collection transport and storage, preparation and manipulation of specimens, and recording, reporting, and interpretation of results. QA also includes other activities designed to improve the quality of investigations e.g., staff training and education, evaluation of the adequacy of the laboratory environment, and maintenance and calibration of instruments. All these activities should be described in a quality manual which is a prerequisite to obtain the certification of the quality system or laboratory accreditation.