In an increasingly complex society there are a number of different population definitions that can be relevant for users, beyond the standard definition used in counting the population. This article describes the enumeration base for the 2011 Census and how alternative population outputs may be produced. It provides a background as to how the questions on the questionnaire were decided upon and how population bases can be constructed from the Census. Similarities and differences between the information collected across the three UK Censuses (England and Wales, Scotland and Northern Ireland) are discussed. Finally, issues around estimating the population on alternative bases are presented.