Sol-gel chemistry allows the synthesis of hybrid organic-inorganic materials. Organic molecules can be used as complexing ligands to provide a chemical control over hydrolysis and condensation reactions, leading to the formation of stable suspensions of nanoparticles. They can also be trapped within the sol-gel matrix in order to provide some optical properties such as photochromism. When chemically bound to the oxide network via Si-C bonds they lead to hybrid nanocomposites that offer new possibilities in the field of materials science. Even biospecies, such as enzymes or antibodies, can be trapped within sol-gel matrices in order to make bio-sensors or bio-catalysts.