The report covers the first five years of operation of the U.S. Public Health Service's National Air Sampling Network. It has been designed to present basic data on suspended particulate matter in the air over the United States. Much of its content consists of tables which present this information. The various text sections reflect the second purpose of this report: to provide the technical information necessary for the establishment of similar or identical sampling stations in community, area, or State Networks throughout the United States by appropriate agencies. For this reason, data concerning the statistical and chemical procedures used in the Network have been presented in greater detail than probably would have been necessary for an evaluation of the results.