Many junior doctors in England are refusing to go to work for the next 24 hours because they are unhappy with government plans to change their pay and working hours.
Emergency operations will still go ahead during the strike, but thousands of day-to-day appointments and less urgent operations have been cancelled.
At the moment it's expensive for hospitals to employ lots of doctors at the weekends because they get paid more for working on Saturdays and Sundays, as well as at night. The government wants to make it cheaper.
Junior doctors are newer doctors, who are still in training. They say planned changes are unfair but the government says they will improve things for patients.
The doctors who are striking say they are worried that working longer hours would make them tired so that they couldn't do things properly, putting patients at risk.
But the government say the plans are fair and it would mean things would be better for people going into hospital, particularly at the weekend.