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 separation of sub-stoichiometric SiOx layers. This size control is realized by different approaches to introducing a superlattice-like structure into the synthesis process, by velocity selection of silicon aerosols, or by the use of electron lithography and subsequent oxidation processes. Nanocrystals between 2 and 20 nm in size with a full width at half maximum of the size distribution of 1 nm can be synthesized and area densities above 1012 cm–2 can be achieved. The role of surface passivation is elucidated by comparing Si/SiO2 layers with superlattices of fully passivated silicon nanocrystals within a SiO2 matrix. The demands on silicon nanocrystals for various applications such as non-volatile memories or light-emitting devices are discussed for different size-controlled nanocrystal synthesis approaches.