Combustion processes often produce solid carbon particles, i.e., soot. These particles may be oxidized to form gas-phase species or released into the exhaust stream, where they can be coated with liquid coatings. These coatings can be comprised of any of a number of components, including unburned fuel, lubrication oil, sulfuric acid, water, and other combustion byproducts. The Particle Diagnostics research program focuses on the developing optical diagnostics for soot particles in combustion environments and combustion exhaust plumes. The goal of this work is in situ measurements of volume fraction, size, composition, and morphology of combustion-generated particles with fast time response and high sensitivity. Measurement techniques are targeted for studies of soot formation and evolution and must be versatile enough to probe particles throughout their entire life cycle. Techniques are developed for detection and characterization of particles in combustion environments from incipient particles that are 2–20 nm in diameter and composed of condensed large organic species to mature soot particles composed of aggregates of carbonaceous primary particles resembling polycrystalline graphite. Diagnostics are also developed for characterizing inhomogeneous exhaust particles.