Disordered systems near quantum critical points

We discuss various aspects of disordered systems at and near quantum phase transitions, which are transitions at zero temperature that are driven by quantum instead of thermal fluctuations. Special emphasis is put on the so-called Griffiths–McCoy regions surrounding these transitions in magnetic systems, where various susceptibilities are found to be divergent at T→0 without the system being critical. The theoretical concepts are explained for one particular example, the diluted Ising ferromagnet in a transverse field close to the percolation transition at zero temperature.