In early 2013, WHO convened an expert group of scientists from 14 collaborating research institutions to update the assessment of the burden of diarrhoeal disease from inadequate water, sanitation and hygiene (WASH) and to reassess the effectiveness of WASH interventions. This group considered evolving and alternative methods for assessing the burden of disease and agreed on a rigorous new approach using meta-regression. In deriving the new figures, the experts incorporated the latest data on use of improved water and sanitation with minor adjustments, and drew upon the results from two new global reviews on microbial water quality and of handwashing practices specially prepared as part of this effort. This document outlines the latest research on the burden of diarrhoea related to inadequate water, sanitation and hygiene (WASH). It is based on a series of articles published in the scientific literature.In bringing together current evidence on exposure to unsafe drinking-water, inadequate sanitation and hygiene, alongside the most up-to-date analysis on the health impacts of interventions, this document contributes to informed policymaking and targeting of resources. It underscores how further progress can be achieved in this unfinished global water and sanitation and health agenda.