Doctors will today take industrial action for the first time in 40 years.
Around 100,000 doctors - members of the British Medical Association - could in theory strike for 24 hours.
Those taking part will be in their surgeries and hospitals, but they will only see patients with urgent medical problems. A&E and maternity units will be operating normally.
What is it all about?
The Government is imposing a new pensions deal that would mean the best-paid doctors contributing 14.5% of their salary - up from 8.5%.
New doctors would also have to work for three extra years, until the age of 68. And the lucrative final salary scheme would be scrapped in favour of a career average pension.
Why are doctors so angry?
Doctors are furious that the Government has ripped up a deal agreed just four years ago. They argue the pension is self-financing, with a surplus already going to the Exchequer.
And they say it's unfair that they are being asked to contribute twice as much as a civil servant on the same salary to get the same pension.
Who is involved?
The BMA represents around 70% of doctors. In a ballot earlier this month more than two-thirds voted for strike action.
What does the Government say?
The Department of Health calculates that doctors will retire on a pension of £68,000 a year - twice the national average salary. It says the pension is fair and not up for negotiation.
Will the strike be solid?
It would seem not. Hospitals are confident that most operations and outpatient appointments will in fact go ahead. And one survey suggested only a quarter of GP surgeries will take part.
What about my appointment?
Patients booked in to affected clinics or operating lists should have been told by now. Everybody else should turn up as normal.