The winners of each group has automatically qualified for the 2022 World Cup finals in Qatar, but three more teams will seal their spot at the tournament via the play-offs.
But how do they work?
Twelve teams have advanced to the play-offs in total, but how they got there was a little complicated.
Ten group runners-up from qualifying were joined by two teams who qualified via the UEFA Nations League overall ranking.
They are the two best group winners from the Nations League who have not advanced via traditional World Cup qualifying - that is as either group winners or group runners-up.
In the play-off draw, 12 teams are drawn into three play-off paths for one-legged semi-finals and finals.
The three winners reach the 2022 World Cup in Qatar.
Scotland v Ukraine
Wales v Austria
Russia v Poland
Sweden v Czech Republic
Portugal v Turkey
Italy v North Macedonia
Wales or Austria v Scotland or Ukraine
Russia v Poland v Sweden or Czech Republic
Portugal or Turkey v Italy or North Macedonia