For the Welsh and Republic of Ireland football teams it all comes down to Monday evening.
England are already through to the World Cup play-offs, and a place for Northern Ireland looks likely.
Scotland also have the chance to finish second in their group, which could give them a place in the play-offs for Russia 2018.
But for Wales and the Republic of Ireland it's all still to play for. Whoever loses, their World Cup dream is over.
On Friday, Wales won in Georgia, but the Republic of Ireland also picked up three points to set up Monday's big game between the two sides.
The winners of the game in Cardiff will finish at least second in Group D.
As the moment Wales are second in the group, and so a draw would be enough for Wales to see them through.
The Republic of Ireland are currently third, with 16 points. But if they beat Wales, they'll leapfrog them.
The group leaders Serbia lost on Friday night, thanks to a last-minute goal from Austria. This means either Wales or the Republic could earn automatic qualification if Serbia slip up again.
Serbia will play Georgia on Monday, and so they also need a win to be certain of reaching their second finals.
Road to Russia: The key dates
17 October: Europe play-off draw (Zurich)
9-11 November: Play-off first legs
12-14 November: Play-off second legs
1 December: Groups are drawn
14 June - 15 July 2018: World Cup
On Sunday, England play Lithuania, and Scotland play Slovenia at 17:00. Northern Ireland play Norway at 19:45.
The big game between Wales and Republic of Ireland will be played at 19:45 on Monday. Will you be watching?