More than a million migrants and refugees have come to Europe in the last year, to try and start a new life.
People coming from countries like Syria, Iraq and parts of Africa often travel by boat from Turkey to get to other European countries.
One of the places that they get to first is Greece, which has been overwhelmed by the large number of people who have arrived there.
Now a new plan to help deal with the huge numbers of people asking for help has started.
People arriving in Greece, from Turkey, will be interviewed by authorities there.
The authorities will decide if they can stay in Greece or if they will have to go back to Turkey.
If the people can't prove that they have to stay in Greece for their safety, they will have to return to Turkey.
Turkey has said it will accept any people who might be returned there from Greece.
In return, other European countries will accept Syrian people who are currently living in Turkey.
But already some people are worried that the new system is not treating all migrants fairly, and that they may be unsafe if they return to Turkey.