This book is designed to provide comprehensive and up-to-date knowledge on the classification and taxonomy of the microbial world. In recent years, the prokaryotic systematics has evolved significantly with increasing understanding of the evolutionary relationships among microorganisms. The book is divided into five chapters: Chapter I presents the foundation of prokaryotic systematics, covering the historical prokaryotic classification, the importance of systematics, and the fundamental methods of taxonomy and identification. Chapter II delves into the modern approach to prokaryotic taxonomy (polyphasic taxonomy: phenotypic, genotypic, and phylogenetic data for the classification of microorganisms). Chapter III provides comprehensive coverage of the major bacterial phyla, their distinctive features, ecological significance, and biotechnological importance. Chapter IV focuses in-depth on the Proteobacteria and explores the characteristics, evolutionary relationships, and ecological characterizations. Finally, Chapter V is dedicated to the study of Archaea, examining the distinctive characteristics of Archaea, their evolutionary relationships, and their ecological characterizations.