REVIEWSHybrid materials have found widespread applications in recent years and are one example for the bottom-up approach to nanotechnology: materials and phases are generated by physical or chemical means from precursor compounds forming nanoscale or nanocomposite materials. This review describes basic principles and several applications of inorganic± organic polymers using organically crosslinked heteropolysiloxanes (ORMOCERs) based on the sequential synthesis of inorganic and organic networks.