Understanding how to predict and describe the existence of these transitions, their characteristics and consequences for everyday phenomena is one of the more important roles of statistical and condensed matter physics. In this chapter, thermodynamics of phase transitions is described, which are ubiquitous in nature and are widely used to realize, improve or extend the materials properties. Examples include magnets, liquid crystals, superconductors, crystals, amorphous solids, and liquid condensation. These transitions occur between equilibrium states as functions of
, etc.; and define the nature of the matters we deal with on a day-to-day basis.