We recall the geometry of porous silicon and the order of magnitude of
some characteristic parameters. We give a brief review of optical exp
eriments and their different interpretations. We focus on quantitative
interpretations and show that an essential concept is confinement in
a quantum wire or box. In particular, the exchange energy of electron-
hole pairs correlated by Coulomb interaction inside a quantum box expl
ains results obtained between 4 K and room temperature. Nevertheless,
the large shift of the main luminescence line for similar porous silic
on but different electrolytes cannot be explained by quantum confineme
nt alone and has to be accounted for by the difference between the die
lectric constants inside and outside the porous silicon. A brief accou
nt of electroluminescence experiments is also given.