  Several basic problems of the theory of quantum phase transitions are
reviewed. The effect of the quantum correlations on the phase transition
properties is considered with the help of basic models of statistical physics.
The effect of quenched disorder on the quantum phase transitions is also
discussed. The review is performed within the framework of the thermodynamic
scaling theory and by the most general methods of statistical physics for the
treatment of phase transitions: general length-scale arguments, exact
solutions, mean field approximation, Hubbard-Stratonovich transformation,
Feynman path integral approach, and renormalization group in the field
theoretical variant. Some new ideas and results are presented. Outstanding
theoretical problems are mentioned.
