Publisher Summary This chapter describes the microbiology of potable water from a historical as well as an ecological perspective. The historical perspective reacquaints with the insights and endeavors of investigators who preceded researchers in the quest for protection and understanding of the microbial quality of water. The ecological perspective is reviewed as it applies to applied ecology, which attempts to explain why and how organisms move from one ecosystem to another and the mechanisms which prevent or enhance such movement. “Ecosystem” is defined as each unique environment, i.e., source waters, treatment plants, or distribution systems. Once this view is adopted, the subject of potable water microbiology becomes a series of discrete yet intimately linked parts. One can imagine the source water, treatment plants, and the distribution system as separate ecosystems linked together by the continuous flow of water from its origin to the consumer. This chapter identifies the critical linkages and how they function in each of these systems. It endeavors to report not only the state of knowledge of microbiology, but also the importance of chemistry and engineering aspects, in each compartment of the potable water system.