Wales last played at a football World Cup 64 years ago, and a win later will put them just one game away from a return.
Fans from across Wales and the world are descending on Cardiff for one of the most important games in decades.
In 1958 they were eliminated by eventual winners Brazil , with Pele scoring the quarter-final winner.
Tonight they play Austria, and a win will see them face Scotland or Ukraine for a place at the 2022 World Cup.
The game kicks off at 19:45 GMT at the Cardiff City Stadium, but fans have been arriving in the capital since the early morning.
“It’s a massive game,” Noel Mooney, chief executive of the Football Association of Wales, told BBC Radio Wales Breakfast .
“It is a World Cup final for us in many ways.”
He added: “We’ve done everything we can preparing for this, obviously it’s now up to the players after they cross that white line.
“They’ve brought a lot of joy, a lot of honour to Cymru… to get to a World Cup would cement their place in Welsh sporting and folklore history.
“I have no doubt the guys will put on a massive performance in for their country.”
Fans are making the journey down to Cardiff from all corners of Wales – and the world, with one, Mark Pickering, making the 24-hour journey from Tokyo, Japan.
THREAD: Setting off from Tokyo to go to Wales’ World Cup play-off semi-final on Thurs. Have to fly Tokyo-Doha-London. 24+ hours travel to LDN & then a train to ???????. I’ll post some travel updates here. #TogetherStronger #TheRedWall #YWalGoch pic.twitter.com/GgLmdx8ERx
— Mark Pickering ??????? (@_MarkPickering) March 23, 2022
Wales captain and star player Gareth Bale has already said this would be his “last chance” to play for Wales at a World Cup.
Wales qualified for their first international tournament since 1958 at the 2016 European Championships, an experience which Bale said can “inspire” the team to qualify for the World Cup.
“We have to give 100% and have no regrets,” he said.
“We can draw on past experiences that have hurt us but also the excitement of what potentially is ahead, so there is a lot to play for.”
Transport for Wales (TfW) have warned of delays for people travelling to Cardiff via train, and said face-coverings remained mandatory on public transport.
Services after the match will be “extremely limited” due to the late kick-off, TfW added.
Cardiff Bus said Leckwith Road, near Cardiff City stadium, will be closed following the match, with buses diverted.