Environmental microbiology might be considered by some to be an ill-defined subject:Where does the environment begin, and where does it end? From Marcus Terentius Varro’s observations regarding unseen “minute creatures” more than two millennia ago to Antonie van Leeuwenhoek’s first glimpse of the “animalcula” beneath his lens, there is no place on Earth—from thermophilic, acidic springs to the air we breathe to the deepest subsurface locations we have yet been able to reach—where people have looked and not found microorganisms of some type. The domain of what may be considered environmental microbiology thus continues to expand beyond the textbook definition of “the study of microorganisms existing in natural and artificial environments.” At the same time, our knowledge of microorganisms is increasing at an ever-more rapid rate as the result of incredible improvements in analytical methodology, especially at the molecular level. When compiling a manual of this nature, therefore, how does one determine what to include and what to exclude? In the end, the editors decided to showcase as much information as possible on some of the most important areas of environmental microbiology, to provide a clear sense of the possibilities presented by the existence of microorganisms in various environments. Further and more detailed information can be found in the wealth of expertly chosen references within each chapter. This edition of the Manual of Environmental Microbiology has been reorganized, with new and updated sections to provide recent information on topics of importance to the field. The General Methodology volume, beginning with Section 2.1, is devoted to methodologies—the “how” of environmental microbial studies from analytical detection to sample collection. The methods presented in this volume are the basis of how we understand the microbial world around us and are used across environmental microbial disciplines and applications.