Many countries undertake a census every 10 years. The census is very comprehensive and involves every household in the county to legally complete the questionnaire about people in that household covering lots of topics including health, identity and education.
Civil registation
Between census, the UK population data is kept up to date by civil registrations of all births, deaths and mariages. This allows for an up to date count of the population to be recoreded.
Immigration Records
Government records, with information on migration, will show the number of people entering and exiting a country from visa application or border agency.
Sample surveys
Scottish household survey is a continuous survey based on a random sample of the population which is cheaper than carrying out a full census.
NHS Records
Data from the NHS records provides health data inbetween census collection
