Carbonaceous particles in the atmosphere consist of two major components — graphitic or black carbon and organic material. The organic component can either be directly emitted from sources (primary organics) or be produced by atmospheric reactions from gaseous precursors (secondary organics). We define soot as the total primary carbonaceous material, i.e., the sum of black carbon and primary organics. The complex set of questions concerning the origin and the chemical and physical characterization of carbonaceous particles has been central to the research of the Atmospheric Aerosol Research group at Lawrence Berkeley Laboratory since the group’s beginning in 1972. This paper will present an overview of our efforts to quantitate the amount of soot in a variety of urban locations. The results will demonstrate that soot is a major component of the carbon aerosol in all locations and can be the dominant component in many.