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. This fourth edition focuses on consolidating and clarifying changes introduced in the third edition and its addenda, especially on good management practices, and incorporates revised or new risk assessments for various chemical and microbial hazards.