Nicknames: Tykes, Reds
Position last year: 10th in League One
Manager: Former Oldham Athletic manager Lee Johnson was appointed Barnsley head coach on a three-and-a-half year deal on 25th February 2015.
The 33-year-old enjoyed a successful playing career spanning seven clubs - Kilmarnock, Chesterfield, Derby County, Bristol City, Hearts, Yeovil and Brighton & HA - and 472 appearances (35 goals).
Johnson became the Football League's youngest boss when he took over at Oldham's Boundary Park in March 2013, aged just 31. He then guided the Latics to League One safety in his first season and helped them to a 15th-place finish during the 2013/14 campaign.
When he took up the job offer at Barnsley, the Tykes were sitting 16th in League One, but his appointment sparked a run of four successive wins, followed by three successive draws to put the club within touching distance of the top six.
However the team were unable to sustain that run of form, and a disappointing sequence of just one win in their last seven games - a 5-0 final day thrashing of Rochdale - saw Barnsley finish seven points shy of a Play-Off spot.
Key Player: Midfielder Conor Hourihane enjoyed a memorable first season at Oakwell following his move from Plymouth Argyle in the summer of 2014.
Born in County Cork, 24-year-old Conor kicked off his career as a youngster at Sunderland before joining up with Roy Keane at Ipswich Town in 2010.
He failed to make the impact he had hoped for at Portman Road and was on his travels again just 12 months later, arriving at Plymouth Argyle. Three years, 125 appearances and 13 goals later, Conor made the switch to Yorkshire, with Barnsley paying around £200,000 to secure his services on a three-year deal last June.
Hourihane's excellent passing and eye for goal saw him quickly establish himself as a fans' favourite at Oakwell, and he crowned an impressive forst season for the club not only finishing as top scorer, but also being voted as Player of ther Year.
Top scorer 2014/15: Conor Hourihane (14 goals)
Head-to-head: There is nothing to seperate the two teams when it comes to the stats, with both Barnley and Millwall recording 20 victories overall and 12 matches drawn from their 52 previous meetings.
The Tykes and Lions have won 19 games apiece in the League, with the former also tasting success once in the FA Cup whilst The Lions were victorious in the only League Cup tie following a replay.
Last time we met: A solitary goal from James O'Brien after just four minutes secured a 1-0 win for the Tykes at Oakwell on Saturday 22nd February 2014.
O'Brien latched on to a Martin Cranie ball, rounded keeper David Forde and slotted in.
Millwall went close when DJ Campbell shot just wide and Steve Morison had a header excellently saved. But Barnsley, who had a Peter Ramage header cleared off the goal-line, held on to secure the victory.
Barnsley: Steele, Ramage, Kennedy, Cranie, Hunt, O'Brien, Frimpong, Jennings, Mellis (Dawson, 45mins), Proschwitz (Shea, 63mins), O'Grady. Subs not used: Turner, Woods, Lawrence, Cywka, McCourt.
Scorer: O'Brien (4mins)
Millwall: Forde, Shittu, Lowry, Fredericks, Malone (Bessone, 85mins), Woolford, Upson (Feeney, 57mins), Williams, Morision, Campbell, McDonald (Jackson, 73mins). Subs not used: Bywater, Dunne, Abdou, Easter.