Microbiology is an extremely diverse discipline and can be a bewildering field to the novice. The traditional books, seem to be exhaustive of microbiologic facts, are too voluminous and detailed to be read by a typical medical student, who is trying to keep up with simultaneously several classes of other subjects. One of the ways to fulfill the goals of the book is to concentrate on understanding concepts— important fundamental ideas or themes that form a framework of the subject. Essentials of Microbiology is the result of the author’s 28 years of experience in teaching medical students. The book is readable and complete enough to meet the students’ needs. The overall objective of this book is to provide students the basics of medical microbiology as well as a complete coverage of the subject. It contains all of the information that is pertinent to the medical students who are studying microbiology keeping in mind their examination. Although the text has been designed to teach undergraduate and postgraguate medical students, it would also serve as a review tool for the individuals who are taking medical examinations and persons working in health-related professions, physicians, and infectious disease scientists. Microbiology has expanded beyond recognition with various medical specialty, and it is not possible for any one book to cover all aspects of medical microbiology in depth. This book is divided into the following seven sections, based on the major disciplines included within microbiology: General Bacteriology, Immunology, Systemic Bacteriology, Virology, Medical Mycology, Miscellaneous, and Diagnostic Medical Microbiology. The chapters themselves are comprehensive yet free of unnecessary detail and provide the reader with a framework for understanding. Mycology and parasitology have continued to flourish and have blossomed into fields of study of their own rights. Therefore, parasitology has not been included in this book which has a sturdy independence. I would be thankful for any comments or suggestions from students, teachers, and all the readers of this book for further improvements in its future editions.