Champions and Challenge Cup draw: Ospreys face holders Saracens
Ospreys will face 2017 champions Saracens, plus runners-up Clermont Auvergne and Northampton Saints in Pool 2 of next season's Champions Cup.
Scarlets will face Toulon, plus Bath and Benetton Treviso in Pool 5.
Cardiff Blues have a tough Challenge Cup draw, with former European Cup winners Toulouse, Lyon and Sale Sharks awaiting them in Pool 2.
Newport Gwent Dragons are in Pool 1 with Newcastle, Bordeaux-Begles and Russian side Enisei-STM.
Ospreys head coach Steve Tandy reflected the magnitude of his side's challenge against the current European and French champions in Saracens and Clermont.
"For supporters, players, or as a coach when you look at these three games, it's exactly what you want," said Tandy.
"I'm sure the supporters are all talking about it, getting excited, and we're no different.
"We want to pit ourselves against the best and that's definitely the case in this group.
"We're excited about this pool and are already looking forward to testing ourselves in a group which is probably as tough as it is possible to get.
"We missed out on this competition last season so we are pleased to be back in it against the very best.
"Given the clubs we are up against it, we'll probably go in under the radar, but like we saw two years ago in this competition, if you win your home games anything can happen."
Scarlets coach Wayne Pivac said: "The make-up of the groups in this year's competition just goes to show the calibre of the competition with no team facing an easy challenge.
"Winning the Guinness Pro12 means that we head into this season's competition as top seed but we face another tough challenge as we take on former champions and Top 14 finalists Toulon once again.
"Bath is a team we faced in the Anglo-Welsh Cup last season but will be a very different side in Europe, as of course will we.
"There's a long-standing tradition and heritage between the two sides and we'll look forward to taking on the likes of former Scarlet Rhys Priestland.
"Treviso is a side we also know well through Pro12 competition. However, as I've previously mentioned it's not an easy place to go and will be mindful of that as we prepare to take them on in Europe."
Newport Gwent Dragons coach Kingsley Jones admitted there were some familiar foes in their group.
"The pool that we have been drawn into is a tough one, with hard opponents," said Jones.
"During the last few seasons we have played Newcastle in a few different competitions so we know a fair bit about each other.
"They have much improved over the course of last season so it should be a good challenge for the squad.
"Once again we will face Enisei which will see us return to Russia, but our toughest opponent will be Bordeaux and the challenge will be winning away, so our focus will be those away games as we will back ourselves at home."
The opening European games will be played on 12-15 October, with the Pool stage reaching its conclusion on 18-21 January.
Teams will play each other twice in the pool stage, with the Pool winners and three best-placed runners-up qualifying for the quarter-finals.
The quarter-finals are scheduled for 29 March-1 April, the semi-finals for 20-22 April, with the two finals being played at the San Mamés Stadium in Bilbao, Spain, on Friday 11 May and Saturday 12 May 2018.
Challenge Cup draw in full
Pool 1: Newcastle Falcons, Newport Gwent Dragons, Bordeaux, Enisei-STM.
Pool 2: Cardiff Blues, Sale Sharks, Lyon, Toulouse.
Pool 3: Gloucester, Pau, Zebre, Agen.
Pool 4: Stade Francais, Edinburgh, London Irish, Krasny Yar.
Pool 5: Brive, Connacht, Worcester Warriors, Oyonnax.
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