Both sides go into their respective matches on foot of impressive preparatory games but on the clear understanding that what they face today will represent a massive step up on what has gone before.
Leinster of course had the distraction of the Felipe Contepomi affair in the lead up to their game against Biarritz but if they can turn adversity to advantage then they will have the use of the Argentine in the knock out stages.
Of the two sides Leinster have the easier group (Sale Sharks and Cardiff Blues) and a win today over Biarritz will put them in pole position. Munster on the other hand, are after Ulster, in the most difficult group (Gloucester Treviso) and there is no harder place to begin a campaign than France.
Munster's preparation was disrupted by weather conditions that saw their departure from Ireland delayed by almost five hours, fog at Shannon meaning they had to ferry most of the squad to Cork by coach. However the extra day allowed for travel compensated and they expect to have up to 3,000 supporters in Stade Pierre Rajon this afternoon.
Munster: Jeremy Staunton; John Kelly, Mike Mullins, Jason Holland, Shaun Payne; Ronan O'Gara, Peter Stringer; Marcus Horan, Frank Sheahan, John Hayes; Paul O'Connell, Donncha O'Callaghan; Jim Williams capt David Wallace, Anthony Foley.
Leinster Lions: Girvan Dempsey; John McWeeney, Brian O'Driscoll, Shane Horgan, Gordan D'Arcy, Christian Warner, Brian O'Meara; Reggie Corrigan capt, Shane Byrne, Peter Coyle; Malcolm O'Kelly, Ben Gissing; Eric Miller, Keith Gleeson, Victor Costello.