The third edition of the "WHO Guidelines for the Safe Use of Wastewater, Excreta and Greywater" has been extensively updated to take account of new scientific evidence and contemporary approaches to risk management. The revised "Guidelines" reflect a strong focus on disease prevention and public health principles. This new edition responds to a growing demand from WHO Member States for guidance on the safe use of wastewater, excreta, and greywater in agriculture and aquaculture. Its target audience includes environmental and public health scientists, researchers, engineers, policy-makers and those responsible for developing standards and regulations. The Guidelines are presented in four separate volumes: Volume 1: Policy and regulatory aspectsVolume 2: Wastewater use in agriculture Volume 3: Wastewater and excreta use in aquacultureVolume 4: Excreta and greywater use in agriculture. Volume 1 of the Guidelines presents policy issues and regulatory measures distilled from the technical detail found in volumes 2, 3, and 4. Those faced with the need to expedite the development of policies, procedures, and regulatory frameworks, at national and local government levels, will find the essential information in this volume. It also includes summaries of the other volumes in the series. Volume 2 of the Guidelines explains requirements to promote safe use concepts and practices including health-based targets and minimum procedures. It also covers a substantive revision of approaches to ensuring the microbial safety of wastewater used in agriculture. It introduces health impact assessment of new wastewater projects. Volume 3 of the Guidelines informs readers on the assessment of microbial hazards and toxic chemicals and the management of the associated risks when using wastewater and excreta in aquaculture. It explains requirements to promote safe use practices, including minimum procedures and specific health-based targets. It puts trade-offs between potential risks and nutritional benefits in a wider development context. Volume 4 of the Guidelines focuses exclusively on the safe use f excreta and greywater in agriculture. Recent trends in sanitation, including ecological sanitation, are driven by rapid urbanization. The momentum created by the Millennium Development Goals is resulting in dramatic changes in human waste handling and processing. New opportunities enable the use of human waste as a resource for pro-poor agricultural development, particularly in periurban areas. Best practice to minimize associated health risks is at the heart of this volume.