The striking photoluminescence properties of porous silicon have attracted considerable research interest since their discovery in 1990. Luminescence is due to excitonic recombination quantum confined in Si nanocrystals which remain after the partial electrochemical dissolution of silicon. Porous silicon is constituted by a nanocrystalline skeleton (quantum sponge)immersed in a network of pores. As a result, porous silicon is characterized by a very large internal surface area (of the order of 500m2=cm3). This internal surface is passivated but remains highly chemically reactive which is one of the essential features of this new and complex material. We present an overview of the experimental characterization and theoretical modeling of porous silicon, from the preparation up to various applications. Emphasis is devoted to the optical properties of porous silicon which are closely related to the quantum nature of the Si nanostructures. The characteristics of the various luminescence bands are analyzed and the underlying basic mechanisms are presented. In the quest of an eficient electroluminescent device, we survey the results for several porous silicon contacts, with particular attention to the interface properties, to the stability requirement and to the carrier injection mechanisms. Other device applications are discussed as well.