Knowledge of the effects of inhaled pollutants on man is the underlying purpose of research in inhalation toxicology. Specifically, the aims of the chapter are to summarize some experimental design issues, illustrate them with study results demonstrating the use of certain procedures, and discuss the importance of particular methodology. Several important factors in the design and implementation of human exposure studies have been addressed by other reviewers. It is not the author's intent to provide a comprehensive review of either methodology or the pulmonary responses of man to air pollutants, neither of which is feasible in a brief chapter. The results of many investigations of ambient or toxic air contaminants have been and will be used in the establishment of air quality standards. Therefore, certain aspects of experimental design, such as subject selection and characterization, description of symptoms, and details of mode of exposure, require specific attention to elucidate the relevance of these studies to ambient exposure conditions.