  Metal--insulator transitions and transitions between different quantum Hall
liquids are used to describe the physical ideas forming the basis of quantum
phase transitions and the methods of application of theoretical results in
processing experimental data. The following two theoretical schemes are
discussed and compared: the general theory of quantum phase transitions, which
has been developed according to the theory of thermodynamic phase transitions
and relies on the concept of a partition function, and a theory which is based
on a scaling hypothesis and the renormalization-group concept borrowed from
quantum electrodynamics, with the results formulated in terms of flow diagrams.
