European rugby returns for the 2016-17 season with Champions Cup holders Saracens taking on three-time winners Toulon in arguably the pick of the opening weekend's clashes.
Elsewhere, French champions Racing 92 welcome Munster, while Scarlets - the sole Welsh representatives in the top-tier competition - take on Sale.
In the Challenge Cup, big-spending Pau welcome Bath to south-west France.
Connacht v Toulouse (17:30)
Leinster v Castres (15:15)
Northampton Saints v Montpellier (17:30)
Toulon v Saracens (15:15)
Scarlets v Sale Sharks (19:45)
Wasps v Zebre (13:00)
Bordeaux-Begles v Ulster (13:00)
Racing 92 v Munster (15:15)
Bayonne v Gloucester (17:00)
Enisei-STM v Worcester Warriors (15:00)
Pau v Bath (20:45)
Timisoara Saracens v Edinburgh (12:00)
* All kick-off times BST.