Inadequate drinking water quality and poor sanitation have remained the world's major causes of preventable morbidity and mortality. In 1996 the OECD called for concerted action to improve the assessment and management of the world's sources of drinking water. This guidance document seeks to respond to this call. It is the product of a shared initiative between the OECD and the World Health Organization. It is a state-of-the-art review that will contribute to the revisions of the WHO's Guidelines for Drinking Water Quality. Assessing Microbial Safety Of Drinking-water has elements of both revolution and evolution. It is revolutionary in that it supports a rapidly emerging approach for a broader, system-wide management perspective. This is based on a risk management framework that has evolved from the traditional indicator concept to include multiple parameters and where consideration is also given to tolerable risk, water quality targets and public health status.