A commentary on the activities of NIOSH in validating its analytical methods was presented. The commentary was a response to an editorial column which stated that 'no formally accepted guidelines or formats exist for the overall process of design, development, optimization and validation of analytical methods'. It was noted that NIOSH has been involved in developing, evaluating, and validating air sampling and analytical methods for measuring workplace contaminants for more than 25 years. This work has been summarized in the current (the fourth) edition of the NIOSH Manual of Analytical Methods. The manual contains 268 air sampling and analytical methods that can be used to determine more than 350 analytes. In addition to NIOSH's work in this area, the European Community has been working to develop standardized approaches for evaluating air sampling methods. The European Standardization Committee, Technical Committee 137, Working Groups 1, 2, and 3 have been developing standards for evaluating sampling methods for vapors, gases, and airborne particulate matter. The Health and Safety Executive (HSE) of the United Kingdom has also been active in this area. The HSE has published procedures in the manual 'Methods for the Determination of Hazardous Substances' for evaluating air sampling methods that are based on pumped and diffusive samplers. The author concludes that the methods that have been developed and evaluated using these procedures and guidelines can be used to evaluate workplace exposures to a variety of chemicals. They are being used by regulatory agencies and private companies to assess compliance with regulatory standards as well as by research organizations investigating health effects with exposure.