It depends on where they are coming from.
Doctors from outside Europe face rigorous tests from the medical regulator, the General Medical Council (GMC).
They sit a written exam, and undergo practical tests of their clinical and communication skills which can involve actors playing the role of patients.
Only then are they put on the medical register which allows them to work as a doctor in the UK. The total cost to the doctor of sitting these exams is £575.
Why is it different for European doctors?
Under European law their professional qualifications have to be accepted by the GMC.
So a doctor who has trained and qualified in a country anywhere in the European Economic Area, or had their qualifications accepted by that country is entitled to practice as long as they can present proof of those qualifications.
The GMC is not allowed to systematically check their language or clinical skills.
The GMC has said it has serious concerns about the effect of these limitations on its ability to safeguard patients.
Who checks these European doctors?
Once they have been put on the register by the GMC they can apply to the NHS in different areas to go on what is called a "performers list".
In Wales that list is held nationally.
In England each area has its own list, with different criteria for inclusion.
Some require a higher standard of proof that a doctor can speak good english for example.
Doctors are also typically required to provide proof that they are insured, an application for a Criminal Records Bureau check and evidence of any qualifications.
In practice once a doctor is accepted onto a performers list in one part of England they are likely to be able to work in another area too.
How many doctors are flying in from Europe to do these shifts?
There are no centrally kept figures, and no way of telling apart doctors coming in to do an out-of-hours shift or those coming to work more permanently in the UK.
The two countries which have provided the largest number of GPs working in the UK are India and Germany.
In 2008, the total number of Indian trained GPs working was 4,942 and German trained was 1,122 doctors.