The fourth edition of "An Introduction To Microbiology" provides students with a firm foundation in basic as well as applied aspects of Microbiology. As with the successful earlier editions, this edition has been updated to keep pace with the rapidly growing field of Microbiology. The value of the book as a learninig tool is enhanced by materials covering different areas of Microbiology. This textbook is appropriate for beginners in Microbiology at the undergraduate and postgraduate levels. The book is student friendly as its text, figures, and tables have been carefully designed to help students understand the concepts and keep them interested in the material. The organization of the book establishes a firm foundation and provides a perspective on real world applications of Microbilogy. The text has a concise and readable style, covering the current concepts , and gives students the knowledge and mastery necessary to understand advances of the future. To help the students who appear for competitive examinations, we have added a section on multiple choice questions.