In 1977 the National Research Council report on ozone and other photochemical oxidants stated a need for comprehensive human experimental studies that were carefully controlled and documented to ensure reproducibility. This statement was understandable, considering that the major portion of the report's section on controlled human studies was devoted to reviews of test methods, protocol designs, review of a scant amount of published data, and recommendations for future studies. The available data on controlled studies through 1975 were limited to some 20 publications. Nonetheless, this data base represented a substantial increase above the information available prior to 1970, and it became evident that exposure to O3 at low ambient concentrations resulted in some degree of pulmonary dysfunction. In this chapter, emphasis has been placed on the more recent literature; however, some of the older studies have been reviewed again. Tables have been provided to give the reader an overview of the studies discussed in the text and provide some additional information about measurement techniques and exposure protocols. Unless otherwise stated, the O3 concentrations presented in the text and tables are the levels cited in the original manuscript.