The "Guidelines for Drinking-water Quality" is one of the longest-standing normative publications of WHO. They provide an evidence-based point of departure for standard setting and regulation as a basis for health protection. They include an assessment of the health risks presented by the various microbial, chemical, radiological and physical constituents that may be present in drinking-water. Where applicable, they derive maximum concentration guideline values for these hazardous constituents.In the spirit of primary prevention, the "Guidelines" recommend pro-active efforts to assess and reduce health risks. They have evolved from a prescriptive document, which established international standards for end-of-pipe water quality, into a normative best practice manual on drinking-water management.The "Guidelines for Drinking-water Quality" are recognized as the UN system's official position on drinking-water quality. The European Commission and Japan use the "Guidelines" as the scientific point of departure for their drinking-water directive and drinking-water quality standards, respectively; the Australian Drinking Water Guidelines are based on the WHO guidelines, while the United States Environmental Protection Agency (USEPA) and Canada's Health Canada actively observe and participate in the WHO guidelines development and updating process.This fourth edition of the "Guidelines" focuses primarily on consolidating and clarifying the changes introduced in the third edition and its addenda, especially those aspects on good management practices, as well as incorporating revised or new risk assessments for various chemical and microbial hazards.