Due to the recent discovery of the room-temperature visible light emission from porous silicon (P-Si), a great interest in P-Si and related materials has arisen in the last decade of the 20th century. Crystalline (c-) Si, at the heart of integrated circuits, has an indirect band gap of 1.1 eV, which limits its application in optoelectronics. The visible light emitting P-Si may open a new field combining Si integrated technology and optoelectronics. This book is a comprehensive review of the recent research and development of porous silicon. Strong visible photoluminescence (PL) and electroluminescence (EL) from P-Si and other forms of silicon nanocrystallites (nc-Si) are reviewed. Several proposed mechanisms for the PL from porous silicon such as quantum confinement, amorphicity and molecular PL are studied. The following issues are covered: mechanisms for the visible light emission, physical structures, studies of the PL and EL, correlation of structure and optical studies, surface physics and chemistry, relationships among various forms (P-Si, a-Si, µc-Si), device applications, future developments.