The greater Los Angeles metropolitan area (the South Coast Air Basin) historically has had a severe air pollution problem whose visible symptom is a dense smoke-like haze mixed with fog (smog). More than 25 years ago, Friedlander and his associates began attempts to characterize the detailed chemistry of smog particles, and to interpret these results in terms of source emissions. Source apportionment was formalized using a “model”, called the chemical mass balance (CMB). The method has been applied periodically since 1968, with major attempts reported in 1975 and 1994, based on large field studies. Despite improvements in particulate sampling and chemical characterization, data availability and major emissions changes, the CMB results qualitatively remain similar. Only broad source categories are identifiable, but they generally make up more than 75% of the total mass concentration. These include primary motor vehicle emissions, suspended (earth) crustal material, and secondary species of ammonium nitrate and sulfate salts. The decoupling of source components in the ambient aerosols suggests that extreme PM events are significantly influenced by factors other than simple mixing and dispersion. Carbonaceous material from combustion emissions and secondary atmospheric production also represent a significant fraction of southern California airborne particles. The carbon fraction includes a complex mixture of soot or black carbon, oxygenated compounds, as well as traces of polycyclic aroma tics. The last group has been of particular interest because of its apparent common origins and its carcinogenic activity. The major contributor to primary carbonaceous material is motor vehicles, The observations also indicate that a significant fraction of carbon derives from contemporary sources such as vegetation or cooking of meat. The CMB has been used to guide regulatory strategies for particulate emissions management. The method could be improved substantially with more extensive source profile development, as well as improved knowledge about the origins of the organic fraction.