This paper reviews new approaches to size controlled silicon nanocrystal synthesis. These approaches allow narrowing of the size distribution of the nanocrystals compared with those obtained by conventional synthesis processes such as ion implantation into SiO2 or phase seperation of sub-stoichiometric SiOx layers. This size control is realized by different approaches to introducing a sperlattice-like structure into the synthesis process by velocity election of silicon aerosols, or by the use of electron lithography and subsequent oxidation processes. Nanocrystals between 2 and 20 nm in size with a fall width at half maximum of the size distribution of 1 nm can be synthesized and area densities above 10(12) cm(-2) can be achieved . The role of surface passivation is elucidated by comparing Si/SiO2 layers with superlattices of fully passivated silicon nano crystals within SiO2 matrix. The demands on silicon nanocrystals for various application such as non-volatile memories or light emitting devices are discussed for different size-controlled nanocrystals synthesis approaches.