Two kinds of measurements often characterize the aerosols resulting from fires, direct optical measurements and measurements of the fraction of specimen mass that becomes particulate mass. The former consists of two components, absorption and scattering, but is measured more typically as the total extinction of a light beam. Such measurements have been made for a long time in connection with other-purpose fire tests, such as ASTM Test Method for Surface Burning Characteristics of Building Materials (E 84) or ASTM Test Method for Heat and Visible Smoke Release Rates for Materials and Products (E 906), or as separate tests, such as in ASTM Test Method for Specific Optical Density of Smoke Generated by Solid Materials (E 662). The latter often is considered the definitive measurement, and is referenced often for regulatory or specification purposes. Recent work at NBS points to a number of shortcomings of this test, including the limited irradiance range, the absence of continuous specimen mass loss measurements, and the small, fixed amount of oxygen available for combustion. The result is the development of both optical and gravimetric soot measuring techniques for the cone calorimeter. The cone calorimeter is a test method for flammability, recently published as ASTM Proposed Test Method for Heat and Visible Smoke Release Rates for Materials and Products Using an Oxygen Consumption Calorimeter (P 190), which comprises a flow-through system, with two specimen orientations, a wide specimen irradiance range, a load cell for real-time specimen mass loss determinations, and modern signal manipulation hardware. Smoke and soot production data have been obtained in the cone calorimeter for some reference materials and for actual furnishings. These data are seen to be free of the difficulties present with the earlier methods. A small amount of full-scale data is also available; a comparison of the bench-scale data against the full-scale performance shows a promising correlation.