Publisher Summary Sol-gel processing of nanocomposite materials has evolved recently because of the generality of this synthetic method. A wide variety of host matrices can be formed under very mild conditions with relatively inexpensive reagents using sol-gel chemistry. Conversion of a sol to a gel using hydrolysis and intermolecular condensation reactions occurs, under proper conditions, even for complex reactant mixtures affording convenient syntheses of host matrices having complex compositions. This flexibility enables the tuning of the pore size, surface area, density, dielectric constant, refractive index, and chemical composition of a host matrix. Inorganic/organic matrices are also accessible, and sol-gel chemistry even occurs within inverse micelles. Formation of the nanoparticulate phase is achieved most commonly through reduction processes for elemental particles or precipitation reactions for chemical substances. A variety of methods are available to initiate particle formation. Nanoparticle growth is usually promoted by thermal annealing, and, for many substances, thermal energy is required for single-crystal formation. This growth process typically affords nanoclusters of the guest phase having a distribution of particle sizes.