This book is intended as an introduction to the theory of phase transitions and critical phenomena. The phase transitions are described by the methods of thermodynamics and statistical physics. On the contrary modern methods for the investigation of outstanding problems in statistical physics, thermodynamics, condensed matter physics and field theory may be partly ascribed to the development of the ideas and theoretical techniques intended to solve the questions of phase transitions. The sophistication of theoretical methods is often an obstacle to the quick understanding of important results currently reported in scientific literature. The main purpose of this book is to present a part of these methods in an intelligible form for students.