This is the third time the country has been in total lockdown in almost a year, although the rules have changed slightly each time.
Like the first lockdown, schools will be closed apart from for vulnerable children, and the kids of key workers.
And Brits are also only being told to leave their house once for exercise, like in the first lockdown.
Similar to the November lockdown, the extremely clinically vulnerable who were asked to shield are being advised to stay at home.
You can read about the full list of rules here, including everything you can and can't do.
But when will the lockdown end? Here is everything we know so far.
When will the national lockdown end?
The new, tougher measures are expected to last until at least mid-February, at the earliest at the end of school half term on February 19.
But we won’t be free of restrictions at that point.
It will instead allow for the return of the more relaxed tier system, where local restrictions are based on five different factors.
But Mr Johnson said people should remain cautious about the timetable ahead.
The end of lockdown will depend on the success of the vaccine, the rollout and the Covid death rate.
The government stuck to the deadline for the November lockdown, which lasted a month.
But Mr Johnson has already confirmed the time scale could change based on what happens next.
Mr Johnson said: “If our understanding of the virus doesn't change dramatically, once again, if the rollout of the vaccine programme continues to be successful, if deaths start to fall as the vaccine takes effect and – critically – if everyone plays their part by following the rules, then I hope we can steadily move out of lockdown, reopening schools after the February half-term and starting cautiously to move regions down the tiers.”
Most likely, areas in England would go back to the four Tier system.