Scarlets, Cardiff Blues, Ospreys and Dragons learn European fate in Champions and Challenge Cups
Scarlets have been drawn against Racing 92, Leicester and Ulster in Pool 4 of the 2018-2019 Heineken Champions Cup.
Cardiff Blues won last season's Challenge Cup but return to the top tier Champions Cup and play Saracens, Glasgow and Lyon in Pool 3.
Ospreys are in European club rugby's second tier and face Pau, Worcester and Stade Francais in Challenge Cup Pool 2.
Dragons were drawn in Pool 1 against Northampton, Clermont Auvergne and Romanian side Timisoara Saracens.
Scarlets reached the semi-finals of the Champions Cup last season before losing 38-16 to Leinster and face Irish opposition again in the shape of Ulster, plus English side Leicester and Racing of France.
Coach Wayne Pivac said: "A semi-final in last season's competition is something that we're all immensely proud of but... getting a good start in these competitions is vital and that's something we failed to do last season.
"We will be looking to improve our performances in the opening rounds, look to build on last season's success and ensure that we remain competitive in both domestic and European competitions."
Saracens won back-to-back titles in 2016 and 2017 and will start as Pool 3 favourites, while French side Lyon and Scotland's Glasgow - familiar foes from the Pro14 - are the others to stand in the way of Blues.
"You get to the Champions Cup because you've really achieved and had a good campaign the previous season, therefore it's important we give a good account of ourselves and we're looking forward to the challenge," said new Blues coach John Mulvihill.
"The boys will be able to take a lot of confidence from what they achieved in the back half of last season and we need to build on that going forward.
"As you'd expect in this competition, it is a very challenging set of fixtures, but the squad will be motivated at the prospect of facing some of the best teams around the continent in one of the best competitions in club rugby."
Ospreys lost their Pro14 play-off with Ulster to qualify for the Champions Cup, but will still face a tough task in the Challenge Cup against two French teams - Pau and Stade - plus English side Worcester, who have Wales wing Josh Adams in their squad.
New Ospreys coach Allen Clarke said: "Pau showed their quality in this competition last season... Stade are one of the traditional giants of French rugby and... Worcester are a club that have invested and recruited, so will have increased expectations for the coming season.
"I know some of the coaching staff and we'll see Ashley Beck coming back to the Liberty which will give it an added dimension."
Three-time Champions Cup finalists Clermont of France and English side Northampton, the 2000 European champions, will provide stiff competition for Dragons, while Romanians Timisoara provide less familiar opposition.
Dragons coach Bernard Jackman said: "When you have Clermont and Northampton in there it could have been a Champions Cup group.
"We are against two teams who have two of the biggest histories in European rugby and to have them coming to Rodney Parade is exciting."
The first round of European games will take place on 12/13/14 October, with the final pool fixtures on 18/19/20 January 2019.
The quarter-finals are set for 29/30/31 March 2019, with the semi-finals scheduled on 19/20/21 April 2019.
The Champions Cup final will be played at St James' Park, Newcastle, on 11 May 2019, with the Challenge Cup final the day before at the same venue.
Champions Cup 2018-19 draw:
Pool 1: Leinster, Wasps, Toulouse, Bath
Pool 2: Castres, Exeter Chiefs, Munster, Gloucester
Pool 3: Saracens, Glasgow Warriors, Lyon, Cardiff Blues
Pool 4: Scarlets, Racing 92, Leicester Tigers, Ulster
Pool 5: Montpellier, Newcastle Falcons, Edinburgh, Toulon
Challenge Cup 2018-19 draw:
Pool 1: Northampton, Clermont Auvergne, Dragons, Timisoara Saracens
Pool 2: Pau, Ospreys, Worcester, Stade Francais
Pool 3: Sale Sharks, Connacht, Bordeaux-Begles, Perpignan
Pool 4: La Rochelle, Zebre, Bristol, Enisei-STM
Pool 5: Benetton, Harlequins, Agen, Grenoble