Evidence for persistent photoconductivity, i.e., electrical conductivity changes existing for a very long time after the excitation of nonmetallic solids by photons, was furnished back in the 19th century and put to practice even before modern solid-state physics had developed. At present, two complementary models are basically used to explain this phenomenon. One involves the trapping of nonequilibrium charge carriers by point centres of localization (traps) which slows down the recombination of electrons and holes generated by light or charged particles. In the other, electrons and holes are also separated spatially and prevented by potential barriers from recombination. Both types of relaxation process are discussed and experimental data, with special emphasis on the charge separation idea, presented.