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The formation of nanoparticles in laboratory hydrocarbon flames is reviewed in terms of particle morphology, chemical composition, and health hazards. The nascent nanoparticles in the nucleation mode have been widely reported in diverse laboratory flames, and are distinguished by their occurrence as singlet particles that form translucent images in transmission electron microscopy (TEM). Their sizes range from about 10 nm or more down to 2 to 3 nm, the limit of resolution of the TEM, and they possess a liquid-like quality. These particles are widely considered to be the precursor stage to the more readily observed carbonaceous aggregates consisting of chained primary particles that are opaque to the electron beam of the TEM. Nanoparticles sampled from the inverse diffusion flame and the particle effluent from diesel engines show a strong resemblance by GCMS analysis, and they contain many of the stabilomer PAHs and their isomers in the 200 to 302 atomic mass range. Many of these chemical species have high relative mutagenicities. Distinctive bimodal particle size distributions can be observed in both flame and engine samples. Recent TEM micrographs of diesel particulates show images of precursor-like nanoparticles, of as yet unknown chemical composition, that are formed in a diesel engine at many operating conditions.