Thousands of them have been on picket lines and the hostilities between the government and the doctors' union, the BMA, show few signs of abating.
There's no question this is a very significant problem for the government. Thirty eight per cent of junior doctors did go to work today, but that figure includes the planned emergency cover, and those non-members of the BMA.
And falling out with the medical profession, allowing relations to have got so bad, potentially risking patients' health, is a serious risk for any government.
There's not much sign though of ministers backing down. In terms that could hardly be stronger, one well-placed government source told me today: "We all know they are going to lose."
And when you listen to the words of the health secretary who says this strike is unnecessary, that the government promised to make changes to the NHS in its manifesto and they're determined to drive them through, it's clear ministers are a very long way from giving up.
And it's worth noting the government does have the option of just imposing the new terms and conditions as contracts are renewed - that could start as early as next month.
More talks are planned for later this week. And ministers are not yet at the stage of deciding to use brute force to impose the changes.
The hope in government is that once today's strike is over, progress might be made in the talks in the two weeks before the second planned walkout.
Neither side, publicly, is willing to give any ground. But there are nerves about the third planned strike, planned for the middle of February, which is expected to be an all-out strike where no emergency cover would be arranged.
There's a belief in government that's a line that no-one wants to cross.
But there is a risk for the government too if public opinion settles on the doctors' side.
PS: In different times, the Opposition would likely have been criticising the government's handling of a situation like this as loudly as possible.
Today has been rather different. Labour has not been making the political running on the issue.
It's worth noting that the shadow health secretary, Heidi Alexander, did not join any of the strikers on the picket line, while others like the shadow chancellor John McDonnell did.
Junior doctors have been out on strike in the first protest like it for decades.