This book describes comprehensively theories and methods of the power system voltage stability. It first introduces the basic theory of the power system and the basic concept and classification of the power system stability and discusses the basic concepts of voltage stability, including the mechanism of voltage stability, and influencing factors of transient and medium-term and long-term voltage stability. This book also describes the elemental characteristics and models of important power system in voltage stability analysis and discusses the theories and methods of analysis on steady, transient and medium-term and long-term voltage stability analysis, respectively. Then, this book introduces the measures to improve the voltage stability. Finally, two examples of voltage stability analysis in engineering applications are introduced.  This book is useful as a reference for engineers and technicians who are engaged in dispatching operation, planning,design and scientific research of the power system, and teachers and students of electrical engineering major in colleges and universities.