Police are warning fans and players to stay calm at Old Trafford in Manchester on Wednesday evening, as Man United take on local rivals Man City.
The first leg of the Carling Cup saw United's Gary Neville and his former team-mate Carlos Tevez have a bit of an argument after the match.
Police have arrested seven people they think were causing trouble at Man City's ground last week.
United will be battling to reverse City's 2-1 advantage at Old Trafford.
They'll need to do this to keep their place in the League Cup.