Phase transitions belong to the oldest known problems in physics. The first microscopically correct descriptions of the phenomenon, however, only materialized at the beginning of the 20th century and the modern theory of phase transitions relies heavily on methods of quantum mechanics and statistical physics. This work considers i.a. gas-liquid, magnetic, and structural phase transitions between different crystal structures. In these applications a realistic modeling of the many body problem in general and correlation effects in particular become important. It turns out that the behavior of physical observables in the vicinity of the critical point remains universal, i.e. independent of the thermodynamic system being considered. The book is aimed at postgraduate physics students and scientists working in the field of Solid State Physics.