Safe water and adequate sanitation are basic to the health of every person on the planet yet many people throughout the world do not have access to these fundamental needs. An important step towards resolving this global crisis is to understand its magnitude: how many people lack access to drinking water and sanitation? To help answer this question household surveys and censuses are conducted every year throughout the world to assess drinking water sanitation and hygiene-related practices at the household level. Because of variations in survey tools attempts to compare the results of one survey with those of another have been fraught with diffi culties. Solving survey comparability problems is crucial if we are to establish accurate trends over time within a country and compare data between countries. For this reason the WHO/UNICEF Joint Monitoring Programme for Water Supply and Sanitation developed a set of harmonized survey questions that resolves the comparability problems of the past. Including the harmonized questions in national surveys and censuses will help countries gain more systematic information on the drinking water and sanitation needs of thei population. With popular use these harmonized questions will make data across international and national survey programmes more comparable which in turn will result in more accurate country regional and global estimates of unmet drinking water and sanitation needs. The core harmonized questions presented in this guide have been adopted by the Demographic and Health Surveys the Multiple Indicator Cluster Survey and the World Health Survey. The purpose of this guide is to encourage even more widespread use of these harmonized questions because accurate information is a valuable tool that can be used to assess progress towards international and national goals and targets. It also allows decision-makers and stakeholders to make evidence-based choices and direct efforts to where they are needed as well as promotes increased investment in the sector.