contains a short historical introduction. Chapter 2, represents an updated review of microbial diversity and systematics. It also provides essential information required for the understanding of the form, function, and systematic relationship of microorganisms. Chapter 3 is devoted to the formation and structure of microbial communities, and deals with this subject both in the evolutionary and successional senses. Chapter 4 describes the interactions between microorganisms, and Chapters 5 and 6 explore the interactions of microorganisms with plants and with animals, respectively. Chapter 7 discusses the quantitative measurement of numbers, biomass, and activity of microorganisms; Chapter 8 examines the influence and the measurement of their environmental determinants. Chapter 9 presents air, water, and soil as microbial habitats and describes the typical composition of their communities. Chapters 10 and 11 contain an expanded discussion of the biogeochemical cycling activities performed by microbial communities. Chapters 12-15 deal with applied aspects of microbial ecology evident in biodeterioration control, sanitation, soil conservation, pollution control, resource recovery, and biological control.