A census is the procedure of systematically acquiring and recording information about the members of a given population. It is a regularly occurring and official count of a particular population. The term is used mostly in connection with national 'population and door to door censuses' (to be taken every 10 years according to United Nations recommendations), agriculture, and business censuses. The term itself comes from Latin: during the Roman Republic the census was a list which kept track of all adult males fit for military service.

The census can be contrasted with sampling in which information is obtained only from a subset of a population. Census data is commonly used for research, business marketing, and planning as well as a base for sampling surveys. In some countries, census data is used to apportion electoral representation (sometimes controversially so - see e.g. Utah v. Evans).

It is widely recognized that population and housing censuses are vital for the planning of any society. Traditional censuses are, however, becoming more costly. A rule of thumb for census costs in developing countries has been $1 USD per enumerated person. More realistic figures today are around $3 USD. These approximations should be taken with great care since a variable number of activities are included in different countries (e.g. enumerators can either be hired or requested from civil servants). The cost in developed countries is far higher. The cost for the 2000 census in the U.S. was estimated to be $4.5 billion USD, more than $15 per enumerated person. Alternative possibilities for retrieving data are being investigated. Nordic countries Denmark, Finland and Norway have for several years used administrative registers. Partial and sample censuses are used in France and Germany.