Phase equilibrium describes the way phases (such as solid, liquid and gas) co-exist at some temperatures and pressure, but interchange at others. First, the criteria for phase equilibria are discussed in terms of single component systems. Then, when the ground rules are in place, multicomponent systems, are discussed in terms of partition, distillation and mixing. The chapter also explains the criteria for equilibrium in terms of the Gibbs function and chemical potential. This book explains the phase, component, degrees of freedom,phase rule, types of phase diagram and one component system, two component system, ternary system with more examples. The book is intended to take chemistry or physics students with no background in the area, to the level where they are able to understand the phase equilibria, and are able to consider potential ares of new research