Both nanocrystal silicon and porous silicon thin films show efficient visible luminescence at room temperature, thus suggesting the possibility of introducing some form of optically active, silicon-based material into integrated circuit processing. In this context, I discuss and analyze the responsible electronic quantum size effects and excited-state photophysics. In a broader context I discuss silicon optical and electronic properties as a function of dimensionality and surface chemistry. As one progresses from trans-polysilane (1D-Si) through puckered sheet polysilyne (2D-Si) to diamond lattice silicon (3D-Si), there is a systematic progression from direct to indirect gap behavior. In 2D-Si the direct and indirect gaps are nearly degenerate, and the electronic properties can be tailored through surface chemical derivatization. It may be that a direct gap material can be found in the hydroxysiloxene materials. Bulk 3D-Si is remarkable for its slow rates of both radiative and nonradiative intrinsic excited-state decay. Nanocrystal Si and porous Si are indirect gap type materials with oscillator strengths that are not markedly increased with respect to bulk Si. Luminescence increases because spatial confinement keeps the electron and hole superimposed and because surface nonradiative rates are also extremely slow. Theory indicates that lowering of the diamond lattice symmetry via physical effects, such as strain or nanometer finite size, creates only a relatively minor perturbation of the 3D-Si luminescence. However, finite nanometer size can increase the indirect gap by as much as 1 eV. Microcrystalline silicon can be an efficient optical cavity.