Nanoparticle aerosols refer to particles smaller than 100 nanometers. Nanoparticle aerosols play an important role in the environment, in global climate, in human health, in materials processing and in nanotechnology. Industry uses aerosol processes for the manufacture of powdered materials of many different kinds including reinforcing fillers, pigments and catalysts and in the manufacture of optical fibers. Aerosol-based processes are a common method for the industrial production of nanoparticles. Aerosols can be defined as solid or liquid particles in a gas phase, where the particles can range from molecules up to 100 µm in size. Aerosols were used in industrial manufacturing long before the basic science and engineering of the aerosols were understood. For example, carbon black particles used in pigments and reinforced car tires are produced by hydrocarbon combustion, titania pigment for use in paints and plastics is made by oxidation of titanium tetrachloride, fumed silica and titania are formed from respective tetrachlorides by flame pyrolysis and optical fibres are manufactured by similar process.