We review recent results concerning the structural properties and the
origin of visible luminescence in porous silicon. The preparation of p
orous silicon is briefly described and current models for radiative re
combination in porous Si are discussed. Special attention is given to
a comparison between the properties of porous Si and those of siloxene
and amorphous Si:O:H alloys, based on vibrational, optical, and spin
resonance spectroscopy.