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Wales will secure a top-two spot with a point and at worst a play-off with a win. They can top the group if they win and Serbia don't, while a draw would be enough if Serbia lose by two goals.

However - a draw might not be enough to secure a place in the play-offs. Really, both sides need to win.

Dafydd Pritchard
BBC Sport Wales at the Cardiff City Stadium

Image source, PA

Permutations, permutations, permutations.

Contemplating the various scenarios for Wales and the Republic of Ireland's qualification hopes can leave you feeling dizzy.

Although a draw would be enough to secure Wales a play-off place if other results go their way, the feeling at the Cardiff City Stadium is that the teams need to keep their destiny in their own hands.

Only a win will guarantee a play-off spot, so the two sides should produce a full-blooded encounter, roared on by what promises to be a raucous sell-out crowd.

Victory for Wales coupled with Serbia failing to beat Georgia would even see Chris Coleman’s side qualify automatically, while an Irish win and a Serbian defeat would take Martin O’Neill’s men to the World Cup as group winners.

It's all amounting to a tense, expectant atmosphere in Cardiff. The calm before the storm.

Juliette Ferrington
BBC Radio 5 live in Cardiff

There was an Irish invasion in the Welsh capital earlier, streets and pubs taken over by the boys (and girls) in green.

Speaking to some of them there’s a real sense they can do this tonight, they certainly don’t feel underdogs.

One told me “We’ve beaten Austria and drawn with Serbia away so Wales is no big deal!”

Others agreed with what both Chris Coleman and Martin O’Neill said yesterday that “It’s gonna be a typical home nation derby, it’s a must win.”

From the scenes at lunchtime and what’s expected in the stadium tonight it’s gonna be a cracker. A great occasion with two sets of passionate fans.

And there's just one change for Wales as Hal Robson-Kanu starts up front instead of Sam Vokes.

Wales: Hennessey: Gunter, Chester, Williams, Davies; King, Allen, Ledley; Ramsey, Lawrence; Robson-Kanu

There are three changes to the Republic side which beat Moldova. Ireland HAVE to win but I'm not sure this is the most attacking line-up in history.

In come Robbie Brady, James McClean and Harry Arter.

Ireland: Randolph; Christie, Duffy, Clark, Ward; Brady, Meyler, Hendrick, Arter, McClean; Murphy

Image source, Reuters

It's as simple as it gets for the Republic of Ireland. It's knockout football.

Ireland must beat Wales to seal at worst a play-off, and will secure first if Serbia lose.

A draw is no good to them.

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Republic of Ireland striker Shane Long has been ruled out of tonight's game.

The 30-year-old Southampton front man played more than 80 minutes of Friday's 2-0 home win over Moldova.

But on the morning of the final Group D match in Cardiff the Republic announced Long's withdrawal with a hip injury.

Nottingham Forest striker Daryl Murphy looks set to start again after scoring both goals against Moldova.

World Cup qualifiers just got interesting.

Tonight is the night for drama. Wales face the Republic of Ireland in a straight knockout.

Image source, Getty Images

One of the two sides could still win the group and qualify automatically for next summer's Russian feast.

One of the two sides could, more realistically, be in the play-offs.

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Both teams could miss out altogether.

The mathematics behind it all are fairly complicated but the essence of the sporting scenario is unconditionally simple for both sets of players:

WIN THE GAME.

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