A census is the procedure of acquiring information about every member of a given population. This term is mostly used in connection with national 'population and door to door censuses' (to be taken every 10 years according to United Nations recommendations); agriculture busy (all agriculture units) and business censuses (all enterprises). 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 only obtained from a subset of a population. As such it is a method used for accumulating statistical data, and also plays a part in democracy (voting). Census data is also commonly used for research, business marketing, planning purposes and not at least as a base for sampling surveys.

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 have for a long time been $1 USD per enumerated person.Fact: date=September 2008 More realistic figures today are around $3 USD.Fact: date=September 2008 These approximations should be taken with great care since a variable amount of activities can be 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. is estimated to $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 censuses ‘Micro censuses' or ‘Sample censuses' are practiced in France and Germany.

Census and privacy

While the census provides a useful way of obtaining statistical information about a population, such information can sometimes lead to abuses, political or otherwise, made possible by the linking of individuals' identities to anonymous census data.

It is typical for census data to be processed in a way so as to obscure individual information. Some censuses do this by intentionally introducing small statistical errors to prevent the identification of individuals in marginal populations; others swap variables for similar respondents. Whatever measures have been taken to reduce the privacy risk in census data, new technology in the form of better electronic analysis of data poses increasing challenges to the protection of sensitive individual information.

Ancient and medieval censuses

The first known census was taken by the Babylonians in 3800 BC, over 5000 years ago. Records suggest that it was taken every six or seven years and counted the number of people and livestock, as well as quantities of butter, honey, milk, wool and vegetables.

Census in Egypt are said to have been taken already during the early Pharaonic period. 3340 BC and in 3050 BC.