Particle nucleation in a thermal plasma reactor occurs as a high-temperature gas undergoes a cooling trajectory. Cooling leads to formation of supersaturated vapours, which causes either homogeneous or ion-induced nucleation. Detailed models have been developed for homogeneous nucleation in a plasma reactor, including discrete-sectional models and moment-type models. The discrete-sectional models are capable of incorporating size-dependent cluster property data as these become available. Calculations in which a one-dimensional cooling trajectory was assumed in a discrete-sectional code indicate that cooling rates play a key role in determining the final particle size. Moment-type models are more computationally efficient, and have been coupled to two-dimensional reactor transport models. A two-dimensional model was compared with experimental results for synthesis of iron particles over a range of conditions in which the volume-mean particle size ranged from roughly 20 nm to 70 nm, with reasonable quantitative agreement for particle size versus reactant feed rate. The major weakness of current models for particle formation in thermal plasmas is the lack of an adequate understanding of ion-induced nucleation. Additionally, there is considerable need for well-characterized experiments in which particle sizes are determined by probe sampling or light scattering.