The 2021 census has begun as authorities try to determine how many people are living in England and Wales - and what the population needs.
Here's our guide to what to expect.
What is the census?
The census is a survey of people and households in England and Wales, and it takes place every 10 years.
It is an official count of every person and household in the country, and gives the government the most accurate estimate of who lives where and what they do.
The aim is to build "a detailed snapshot of our society", which helps ministers and local authorities plan and fund local services such as education, doctors' surgeries and roads.
Run by the Office for National Statistics (ONS), the census asks questions about you, your household and your home.
When is the 2021 census taking place?
For people living in England, Wales and Northern Ireland, this year's census day is Sunday 21 March.
But for households in Scotland, the census has been pushed back to March 2022 because coronavirus restrictions mean preparations haven't taken place, the National Records of Scotland said.
What sort of questions are included?
Census questions ask about every person living at the property, including their age, race, occupation and relationship status.
For the 2021 census, preparation began several years ago before a white paper was brought to parliament in 2018, followed by a rehearsal the year after.
Background work included user and topic research, testing and evaluation and trying out ideas on people at census events.
Will I get fined if I don't complete it?
Possibly - but you'll be given chances to avoid the penalty.
By law, you must complete or be accounted for in the census and anyone who doesn't fill in the census will be contacted by a census officer.
They'll encourage you to complete it and even help people access any support needed to fill in the form.
If you still don't return or submit a completed census, you will be committing a crime and you will be contacted by the non-compliance team.
If prosecuted, you may have to pay a fine of up to £1,000 plus court costs.
Providing false information can also lead to a fine.
The field officers will be wearing PPE and won't need to enter your house.
Do I have to answer every question?
You do not have to answer the questions marked as voluntary.
People staying in the UK for under three months do not have to fill out a census, but everyone else should be covered by one - including students and those who have gone abroad for under a year.
What's different about the census this year?
A paper form is delivered to all households, but people are being encouraged to complete the form online this year.
For the first time, there will be voluntary questions on gender identity and on sexual orientation for over-16s.
In the UK, there are no official figures for those who identify their gender as different from the sex registered at birth.
Gathering this information will help develop policy, provide services and improve equality, the ONS says.
When will the 2021 census details be released?
We'll have to wait until March next year to read the initial findings from this year's census, while the final release is another 12 months later in March 2023.
Those data dumps will allow us to access information down to a local authority level, but personal details, including anything that could be used to identify people, are kept under wraps for a century.
An ONS spokesperson said: "Like all our data collections, at the heart of the census is keeping information safe, confidential, secure and private - no one can find out individual's details for 100 years."
So, the full census won't be available until 2121, but on the bright side, anyone keen to learn more about their family history will be able to read 1921's full version once it is published online early next year.
Is there a better way of doing it?
Yes - according to Professor Sir Ian Diamond, the UK's national statistician.
He said last month that he is looking at cheaper and more up-to-date alternatives and researching whether data from other sources such as the Ordnance Survey, GP lists, council tax records and driving licence details could replace it.
Combined with regular, large-scale population surveys, this could provide better and more detailed information in a cheaper and more timely way, he said.