In this paper we review our recent results of in-situ luminescence studies of semiconductor electrodes. Three classes of materials are considered: single crystal compound semiconductors, porous silicon and semiconducting oxides doped with luminescent ions. We show how photoluminescence (PL) and electroluminescence (EL) measurements can give information about the optical and optoelectrical properties of the solid and about mechanisms of surface reactions. The relevance of time- and frequency-dependent measurements, which have been largely neglected, is stressed.