The Atmospheric Lifetime Experiment is designed to determine accurately the atmospheric concentrations of the four halocarbons CFCl3, CF2Cl2, CCl4, and also N2O with emphasis on measurement of their long-term trends in the atmosphere. Comparison of these concentrations and trends for the four halocarbons with estimates of their industrial emission rates then enables calculations of their global circulation rates and globally averaged atmosphere lifetimes. The experiment utilizes automated dual-column electron-capture gas chromatographs which sample the background air about 4 times daily at the following globally distributed sites: Adrigole, Ireland (52¿N, 10¿W); Cape Meares, Oregon (45¿N, 124¿W); Ragged Point, Barbados (13¿N, 59¿W); Point Matatula, American Samoa (14¿S, 171¿W); and Cape Grim. Tasmania (41¿S, 145¿E). We review the climatology of these ''clean air'' sites and their ability to describe the global air mass. The instrumentation and methods for data acquisition and processing are then described. An overview of the data obtained and the trends derived during the 3-year period from July 1978 through June 1981 for each of the five species being measured is presented. The comparative behavior of the species with latitude and time is emphasized. The global average surface concentrations of CFCl3, CF2Cl2, CH3CCl3, CCl4, and N2O are increasing at annually averaged rates of 5.7, 6.0, 8.7, 1.8, and 0.2% per year, respectively, at the midpoint of the 3-year period of the measurements.