The Nations League begins this week as Northern Ireland welcome Bosnia and Herzegovina to the National Stadium at Windsor Park this weekend.
The Republic of Ireland, Scotland, England and Wales also begin the new international cycle with Nations League matches during the September international break.
Here, we explain how the new competition will work.
What is the Nations League?
The tournament involves all 55 UEFA member countries and aims to replace meaningless friendlies with increased competition.
When does the first Nations League tournament take place?
It starts in September and continues across the October and November international breaks. The finals will be played in June 2019.
What is the tournament format?
The 55 competing nations have been placed into four tiers based on their current UEFA ranking and each tier is split into four leagues - A, B, C and D - made up of three or four teams. Between September and November the teams in each league will play each other home and away.
The team finishing top of each group is promoted to a higher league and the team finishing bottom of each group is relegated to a lower league, except in the lowest tier (D).
The four teams who win their respective League A groups (the bit England are involved in) will qualify for the knockout finals in June 2019 - semi-finals, final and third-place play-off - to decide the Nations League winner. One of those four teams will act as the hosts of the event - with a decision on that to be taken in December.
Who have the home nations and the Republic of Ireland been drawn to play against?
England will play in the top tier (League A) and have been drawn to play World Cup finalists Croatia, plus Spain, in Group Four. England host Spain on Saturday (September 8) at Wembley.
Wales and the Republic of Ireland will play in the second tier (League B) and have both been drawn in Group Four alongside Denmark. Wales host the Republic on Thursday (September 6) and play in Aarhus on Monday, September 10.
Northern Ireland will also play in League B and have been drawn to face Bosnia-Herzegovina and Austria in Group Three. Northern Ireland play Bosnia on Saturday (September 8) in Belfast.
Scotland are in the third tier (League C) and they have been drawn to play Israel and Albania in Group One. Scotland play Albania at Hampden Park on Monday, September 10.
Will there still be a regular qualifying campaign for Euro 2020?
Yes. The Euro 2020 qualification draw takes place in Dublin in December with all the matches due to be played between March and November 2019.
The top two teams from 10 groups will automatically qualify for the finals. That will account for 20 of the finalists. The final four places are where the Nations League comes in again.
So the Nations League can be a route to qualification for the Euros?
Yes. The four group winners from each league - A, B, C and D, so 16 teams in all - will play off in March 2020 in a bid to secure one of the four remaining places. The format will be one-off play-off semi-finals, followed by a final, per league, with the winning team from each of leagues A, B, C and D taking the last four spots at the Euros.
If a country has already qualified by the conventional route, their place will go to the next highest-ranked team from their respective Nations League tier.