From Jan.1 British and EU citizens will be confronted with the reality of Brexit as the transition period ends and borders done away with decades ago return.
From that date, Britons will be treated by the EU as "third country" nationals, no longer enjoying freedom of movement to work, study or retire across the European Union and associated states.
Britain in turn will process EU nationals at its borders as it does other non-UK passport holders.
EU citizens proving residence in Britain, or Britons already living in a European Union country, before the end of this year will theoretically retain their rights under a Withdrawal Agreement struck in late 2019.
Tourists will see some immediate changes - apart from the evolving coronavirus restrictions already crimping travel - but both sides have agreed that travel will be visa-free, as long as the other side keeps it that way.
But the EU will stop British passports being used in its automated e-gates, potentially meaning longer queues at manned passport booths.
Britons must hold passports still valid for at least six months and will be limited to EU stays of 90 days in a rolling 180-day period.
They will also need to show travel insurance coverage, sufficient funds and a return ticket on request.
Europeans entering Britain can use a national ID card until October, after which only passports will be accepted, for stays of up to six months.
EU passport holders will be able to continue using British e-gates under current guidance.
Those with criminal records may be banned and non-European family members of a European may need a visa, depending on nationality.
The UK treats Irish citizens separately from other EU nationals under a bilateral arrangement dating back nearly a century that allows continued freedom of movement between Britain and Ireland.
Europeans will be able to keep using EU pet passports as long as rabies vaccines are up to date.
Border control for business travelers is one of myriad issues yet to be worked out between the EU and the UK.
In the EU, Britons attending conferences or meetings likely will be exempt from visas where they do not receive payment or provide services.
However for other UK business travelers, including posted workers and the self-employed, a visa and/or a work permit may be imposed in line with each individual EU country's laws.
There will also be tax and social security considerations.
Certain services or company ownership in those countries may be off-limits to non-EU citizens or residents or those lacking national licenses, and customs declarations may be needed for goods carried in.
Without an EU-UK deal, Britain will likely apply its current rules for those coming from favored nations, meaning a visa needed for work but not for short stays to attend a conference or training.
Those same rules would require EU citizens with a job offer to prove English-language skills and a minimum salary dependent on whether the position is skilled (26,500 pounds, equivalent to 29,600 euros or $35,000) or a shortage occupation (20,480 pounds, 22,800 euros).
From January, EU students going to Britain will need a visa for courses longer than six months, and will have to pay steeper tuition fees - four times as much for degrees such as medicine or MBAs in prestigious universities.
UK universities fear that that hefty burden will force many European students to choose EU institutions - some of which are free - instead, blowing a big hole in their finances.
They also say they are already being shunned for research projects led by EU universities.
According to UK parliament research, there were 143,000 EU students in British universities in the 2018 to 2019 school year.
International students have made Britain the second-most popular education destination after the US, and they injected 25.8 billion pounds (29 billion euros, or $34 billion) into the UK economy in 2015.
Without an EU-UK deal, British students will be excluded from the Erasmus+ program offering subsidized exchanges to EU countries.
British students wanting to go to EU universities will encounter higher fees in some countries as well as visa requirements that in many cases will curb their right to work.
For the estimated 1.3 million Britons living in the EU and the 3.6 million EU citizens living in the UK before the end of the transition period, their rights to stay are protected under the 2019 Withdrawal Agreement.
Those wanting to emigrate elsewhere in the EU after Jan.1 will find a very different situation.
Britons, for example, have long favored Spain, France, Germany and Italy to set down new roots as workers or retirees.
But the end of freedom of movement will see them having to jump through the same hoops as other "third country" nationals, which often include health insurance, income and language requirements.
Even Britons settled under the Withdrawal Agreement will no longer have automatic rights to a different EU country, and will face national immigration laws if they want to do so.
Britain, for its part, is bringing in a points-based system from 2021 that will make it significantly harder for Europeans to move there.
Age, English language ability, funds, the requirement to pay a health surcharge will all be evaluated, with caps on some of the immigration channels.