The author combines his knowledge with reporting of accurate update information to illustrate the voltage stability indices and compared how to distinguish numbers of these indices in view of theirs similarity, functionality, applicability, formulation, merit, demerit, and overall performances. This book will serve as a valuable guide for the typical reader. That the readers had in mind were researchers, engineers, planners, and other professionals involved in the assessment of voltage instability in electric power system. The prerequisite for this book is suggested 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.