A summary of a lecture delivered at the XVII Ural International Winter School on Physics of Semiconductors (Ekaterinburg, 2008) is presented. The basic concepts of quantum corrections to the conductivity of two-dimensional systems are briefly laid out. The interference quantum correction and the Altshuler-Aronov correction due to the electron-electron interaction are examined. Particular attention is devoted to systems with spin-orbit interaction. The main ideas are illustrated with experimental results.