Nanocomposites using organic polymers and inorganic nanoparticles will play an important role in applications requiring high visible transparence and mechanical flexibility. A unique class of materials is created when the elastic properties of polymers are combined with the inherent hardness of crystalline nanoparticles. Transparent polymers are generally more flexible than glass materials and exhibit a moderate range of optical, electrical and mechanical properties. The incorporation of inorganic nanoparticles in the polymer system significantly increases the physical parameter range beyond that of the host polymer and allows material properties to be engineered for specific applications. Of particular interest to our group are thin-film nanocomposites on polymer substrates in which strain domains are well matched.True nanoparticles are significantly smaller than visible light wavelengths and, therefore, generally do not cause significant optical scattering when incorporated in a polymer. Careful nanoparticle selection can influence refractive index, electrical conductivity, UV absorption, magnetism and a host of other properties.