The discipline of microbiology that deals with an amazingly diverse group of simple organisms, such as viruses, archaea, bacteria, algae, fungi, and protozoa, is an exciting field of Science. Starting as a purely descriptive field, it has transformed into a truly experimental and interdisciplinary science inspiring a number of investigators to generate th a wealth of information on the entire gamut of microbiology. The later part of 20 century has been a golden era with molecular information coming in to unravel interesting insights ofthe microbial world. Ever since they were brought to light through a pair of ground glasses by the Dutchman, Antony van Leeuwenhoek, in later half of 17th century, they have been studied most extensively throughout the next three centuries, and are still revealing new facets of life and its functions. The interest in them, therefore, continues even in the 21 st century. Though they are simple, they provide a wealth of information on cell biology, physiology, biochemistry, ecology, and genetics and biotechnology. They, thus, constitute a model system to study a whole variety of subjects. All this provided the necessary impetus to write several valuable books on the subject of microbiology. While teaching a course of Microbial Genetics for the last 35 years at Delhi University, we strongly felt the need for authentic compiled data that could give exhaustive background information on each of the member groups that constitute the microbial world.