This book brings together scientists and engineers with diverse backgrounds to share their recent advances in the preparation, characterization and theoretical modelling of microcrystalline and nanocrystalline semiconductors whose properties are influenced by size and boundary effects. Topics include: current status and future prospects of research in microcrystalline and nano-crystalline semiconductors; theory of semiconductor clusters, nanocrystals and porous silicon; synthesis and properties of luminescent Group IV clusters, nanocrystals and nanostructures; synthesis and properties of other Group IV clusters, nanocrystals and nanostructures; Groups III-V and II-VI, and metal sulfide, iodide, and oxide nanocrystals and nanostructures; synthesis, surfaces and structure/property relations of luminescent porous semiconductors; photoluminescence and other optical properties of porous silicon; biochemical, photochemical, photoelectronic, and optical device applications of nanocrystal and porous semiconductors; electroluminescence and light-emitting devices of nanocrystal and porous silicon; preparation, properties and applications of nanocrystalline, microcrystalline and microstructured thin films together with polycrystalline silicon and other crystalline systems.