Image source, PA

Millwall are one win away from returning to the Championship at the first attempt following relegation last season.

Neil Harris' side finished fourth, having been in and around the play-off positions since the start of 2016.

A 3-1 first-leg win at Bradford set up their semi-final win, which was completed with a 1-1 draw at The Den.

Image source, PA

Barnsley, who were bottom of League One in December, won 4-1 at champions Wigan to clinch the final play-off spot on the last day of the season.

The Tykes thrashed Walsall, who had finished third, over two legs to secure a second Wembley appearance in the space of eight weeks.

Barnsley beat Oxford in the Johnstone's Paint Trophy final on 3 April.

That late change for Millwall - while being a blow - is not entirely unexpected for Neil Harris' side.

They were due to check on Byron Webster, who had been struggling with a hamstring problem, before kick-off. It's a straight swap and tactically, they remain 4-4-2.

Will that have any effect on the Millwall squad as they prepare for the biggest game of their season?

Image source, Rex Features

Image source, Getty Images

So how did the Tykes and Lions' seasons compare, statistically?

Well, fourth-placed Millwall edged things over the course of the year but, given Barnsley's poor first half of the campaign, they'd probably argue that they have been the better of the two sides since Christmas...

Just under 30 minutes to kick-off now, and fans continue to arrive at Wembley...

Image source, Getty Images

Image source, Getty Images

After Christmas, Barnsley's campaign transformed dramatically as they switched from relegation contenders to promotion hopefuls in a matter of weeks.

A run of seven straight league wins between 28 December and early February saw the Tykes climb up the table and they finished the regular season with just two losses in 16 games, to claim the final play-off spot.

Meanwhile, Millwall also enjoyed an improved second half to 2015-16, as a run of just one loss in 13 league games propelled them into the promotion hunt and they ended the regular season with four straight wins to finish fourth, seven points above Barnsley.

Image source, Getty Images

Image source, Gett

To call Barnsley's 2015-16 a season of two halves is a huge understatement because, after a run of eight consecutive league defeats, they were bottom of the League One table in December.

By Christmas, they were still in the relegation zone, while Millwall occupied 11th spot, 10 points above Barnsley and six points below the play-off places.

The London club made an inconsistent start but slowly began to improve over the course of the campaign.

Sheffield United was represented at Wembley today - doubling the South Yorkshire contingent - by young footballers who beat Peterborough United before the main event.

The competition is for under-11s and more than 16,000 children took part in the group stages.

Unfortunately rights issues mean we cannot stream live BBC local radio commentary of Football League games online but you can listen in your local area on your radio.

BBC Radio London and BBC Radio Sheffield are both live from Wembley throughout the afternoon.

From 15:00 BST you'll also be able to listen to BBC Radio 5 live's commentary - which will be available via the 'Live Coverage' tab at the top of this page.

Image source, Getty Images

The League One play-off final of 2015 was a one-sided affair, with Preston easing to a 4-0 win over Swindon at Wembley.

Jermaine Beckford scored three times for North End, becoming only the third player to score a hat-trick in a play-off final.

The other two players to do so? If you said Clive Mendonca and Scott Sinclair, well done.

Image source, Getty Images

Saturday saw the first of this Bank Holiday's weekend's trio of Football League play-off finals and it was Hull City who won promotion to the Premier League.

Mohamed Diame's sweet second-half strike was enough to secure an immediate return to the top flight for Steve Bruce's Tigers.

Defeat means Sheffield Wednesday, who had finished nine points below Hull in the league table, now face a 17th year outside the top flight.

Are you on your way to Wembley? Watching the match from the comfort of home? Or maybe you're following the action from overseas?

Wherever you are, I'd love to hear your thoughts on today's game. Get involved via #bbcfootball on Twitter.

Image source, Rex Features

Interim boss Paul Heckingbottom is hoping to guide Barnsley to the second tier and end the club's two-year stay in League One.

And Neil Harris' Millwall are attempting to complete an immediate return to the Championship, following relegation last season.

I'm sure both of the 38-year-old men - who are in their first senior manager jobs - would agree, this is the biggest match of their managerial careers so far.

Rob Stevens
BBC Sport at Wembley

Image source, PA

"The trip to Wembley on the tube was pretty lively.

"Fans of both clubs were in good voice but the Millwall fans in my carriage definitely shaded it. Lions fans love a trip to Wembley, having had back-to-back play-off finals in 2009 and 2010 and an FA Cup semi-final in 2013. Expect them to be in good voice after a loud night at The Den in the second leg of their semi-final against Bradford.

"The Barnsley supporters milling around on Olympic Way have happy memories of their last trip to the national stadium, having won the Football League Trophy last month. Can they recreate that atmosphere?"

Barnsley (4-4-2): Davies; Scowen, Mawson, Roberts, Williams; Isgrove, Hourihane, Brownhill, Hammill; Winnall, Fletcher.

Subs: Townsend, Nyatanga, Chapman, Toney, McCourt, White, Watkins.

Millwall (4-4-2): Archer; Edwards, Webster, Beevers, Martin; Taylor, Thompson, Abdou, Ferguson; Gregory, Morison.

Subs: Forde, Craig, Nelson, Williams, Upson, Onyedinma, O'Brien.

The team news is in and both sides are unchanged from their play-off semi-final second legs.

Stand by for the all-important team news from Wembley...

Tom Garry
BBC Sport

Thanks Trevor - good afternoon all.

It's set to be a bumper Championship season in 2016-17: Newcastle, Aston Villa, Derby County, Nottingham Forest, Norwich City....I could go on.

Today, Barnsley and Millwall go head-to-head for a place in surely the most competitive, unpredictable league in Europe.

Which of them will join Wigan Athletic and Burton Albion in winning promotion to the second tier?

Kick-off is in around an hour's time - stay with us to find out.

Radio Sheffield get to talk to fans who have made it to Wembley from further away, France and Australia in fact.

Mind you there's plenty of people at the game from South Yorkshire and London too.