The search for superhard materials considered so far mainly metastable solids, such as diamond, c-BN and C3N4 and single- and polycrystalline superlattices. More recently, novel superhard nanocrystalline composites have been developed which appear superior to these materials in terms of stability, resistance against oxidation and relatively simple preparation by means of plasma CVD. The present status of the theoretical concept for their design and the experimental results which support it are briefly summarized.