Since 1801 a census has been carried out once every ten years to count the population, and get a comprehensive picture of life in the United Kingdom.
The data collected helps the government to develop policies, plan and run public services, and allocate funding to different areas of the country.
Taking part in the census is a legal requirement, and anybody who refuses can be fined up to £1,000.
The census took place yesterday - Sunday 21 March - with households across the country answering questions on their age, gender, occupation, ethnicity, religion, sexuality, travel, health, and more.
This year's census is particularly important, as it will allow the government to understand how the coronavirus pandemic has affected people's lives.
What happens if you didn't fill out the census?
People who have not filled out the census will receive reminder letters in the coming weeks.
You can also expect to be visited by a census officer who will encourage you to complete it. If you need any support, or if you are having problems accessing the form, they will be able to help you.
You can complete the census online, or you can request a paper copy if you would prefer.
If you still don't submit your completed form, you will be visited by the Census Non-Compliance team, and you may be prosecuted.
The census is not optional, and you can be fined £1,000 (on top of court costs) if you do not take part.
The deadline for the census was yesterday - Sunday 21 March - so you should complete it as soon as possible if you haven't already.
The information you provide on the census will be kept confidential for the next 100 years.
If you want to complete the census separately from the people you live with, so they cannot see your answers, you can still request an individual access code.
No one in your house will know you've done this, and you will be able to answer the questions about yourself secretly.
These answers will override the information provided about you by your household.
