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This article begins by discussing the history of scientific developments on which the understanding of water quality was based. It mentions how the discipline of bacteriology gradually evolved to microbiology during the first half of the twentieth century, the identification of Giardia in the 1960s, the characterization of Cryptosporidium as an “emerging” organism in the 1980s, the burgeoning of organic chemicals production in the first half of the twentieth century, widespread environmental contamination from the pesticide DDT, the development of the Ames test in 1973 for rapid screening of carcinogenicity in chemicals, the development in the 1980s of gas chromatography‐mass spectrometry analytical technology, the detection of previously uncharacterized hydrophilic categories of organic contaminants such as “xenobiotics” that include endocrinologically active compounds and the PPCPs (pharmaceutical and personal care products). The emergence of molecular epidemiology is mentioned along with the formation of such disinfection byproducts as NDMA, chlorate, and bromate from the recent shift toward combining residual chlorination and alternate disinfectants. The article focuses on source protection as the most practical response to emerging contaminants, and mentions how emerging contaminants may dictate integration of a broader range of components in treatment planning, design, and implementation.