A monitoring network was established in metropolitan Atlanta, Georgia, to measure fine particulate mass (PM2.5) and composition for approximately one year, beginning in March 1999. Annual average mass concentrations ranged from 19.3--21.2 ¿g/m3, maximum 24-hour concentrations were 44.3--51.5 ¿g/m3, and maximum 1-hour concentrations were 73.3--87.9 ¿g/m3. Peak mass concentrations occurred typically (but not always) at the Tucker site, which was located approximately 12 miles ENE of downtown. Mass concentrations varied significantly with season and time of day. Approximately 75% of the mass was identified using various chemical speciation analyses. Of the identifiable portion, carbonaceous material, sulfate, ammonium, and nitrate were the largest constituents.