All governments need detailed information on their populations to design and run their health systems effectively. Most countries collect a large part of these data through a census; an exercise often held once every decade that aims to count every inhabitant of every household. With growing populations, the census becomes progressively more difficult and costly. Some countries are moving towards an alternative; combining data from existing population-based registers – such as those that collect births, deaths and addresses – with data from other registers, such as employment and education.