The understanding of power system voltage stability has become increasingly important due to day by day increase in electricity demand and liberalization policy of electricity markets. Therefore, voltage stability has become significantly important during the past decades. Both voltage stability formulation and indices are covered in this book along with an easily comprehensible manner and detailed exposition of the voltage stability indices' fundamental. However, the content of this book is considered serviceable in advanced level. The author combines his knowledge with reporting of accurate updated information to illustrate the voltage stability indices and compares how to distinguish numbers of these indices in view of their similarity, functionality, applicability, formulation, merit, demerit, and overall performances. This book will serve as a valuable guide for the typical reader, specifically for researchers, engineers, planners, and other professionals involved in the assessment of voltage instability in electric power system. It is suggested that the reader have the basic knowledge of power system analysis and voltage stability subjects. The authorship methodology of this book had been based on the reference book style.