Chemical nucleation involves cluster growth by chemical reactions. In the case where clusters grow via a simple sequence of reversible chemical reactions, a summation expression for the steady-state nucleation rate can be derived. However, in many cases the chemical pathway to cluster growth is more complicated, and requires solving a set of species population balance equations that depend on the specific chemical system. Two examples are considered: soot nucleation in hydrocarbon combustion and nucleation of silicon particles in thermal decomposition of silane. In both cases, chemical kinetic mechanisms have been developed that allow for numerical simulations of particle formation. Soot nucleation is believed to proceed through the formation of polycyclic aromatic hydrocarbons. Models have been developed for the formation of the first aromatic ring and for subsequent growth, either through reaction with small molecules or by coagulation. Silicon nucleation from silane involves a large set of silicon hydride species, which can be grouped into classes according to their structure and reactivity, facilitating estimates of their free energies and reaction rate constants.