Preview: Champions Cup, pool-by-pool
Season overview: Quins snatched the last Champions Cup qualification place and have had a mixed start to this campaign. Jack Clifford's injury is a blow.
History: Despite winning the Challenge Cup a record three times, Quins have not managed to translate that success to the Champions Cup. This is their first time in Europe's elite competition since 2014/15.
Key player: Chris Robshaw.
Opening three games: La Rochelle (H), Wasps (A), Ulster (H).
Season overview: Stunned the TOP 14 by finishing first last season, but were undone by an untimely red card in the play-off semi-final against RC Toulon.
History: La Rochelle are making their first ever appearance in the Champions Cup, having enjoyed their best-ever European campaign last season in reaching the Challenge Cup semi-finals, where they lost narrowly to Gloucester Rugby.
Key player: Alexis Bales.
Opening three games: Harlequins (A), Ulster (H), Wasps (H).
Season overview: New head coach Jono Gibbes has made a decent start, although the shock PRO14 defeat to Zebre will have been a reality check.
History: The 1999 winners have never failed to qualify for Europe's elite competition, but haven't made it past the Pool stage for the last three seasons, let alone threatened to repeat their run to the 2012 final.
Key player: Jacob Stockdale.
Opening three games: Wasps (H), La Rochelle (A), Harlequins (A).
Season overview: Last season's domestic first-place finish and quarter-final berth in the Champions Cup seems a long time ago for Wasps, who have lost four of their opening six Aviva Premiership games.
History: The great Wasps side of the early 2000s lifted the European Cup trophy twice in that period, but the last three years have seen one semi-final and two quarter-final exits.
Key player: Nathan Hughes.
Opening three games: Ulster (A), Harlequins (H), La Rochelle (A).
ASM Clermont Auvergne
Season overview: TOP 14 champions and Champions Cup finalists last season, Clermont have started surprisingly slowly this term, with four defeats in their first seven domestic games.
History in competition: Three-time runners-up (in 2013, 2015 and 2017), the double Challenge Cup winners (1999 and 2007) will be eager to break their duck in the Champions Cup this season.
Key player: Morgan Parra.
Opening three matches: Ospreys (A), Northampton (H), Saracens (A).
Season overview: A rejuvenated backline has brought a good start to the season in the Aviva Premiership and offers cause for optimism.
History in competition: Not since 2011 have Saints mounted a sustained shot at the European title, when they lost 33-22 to Leinster Rugby in the final at the Millennium Stadium.
Key player: Piers Francis.
Opening three matches: Saracens (H), Clermont (A), Ospreys (H).
Season overview: A youthful squad has been boosted by James Hook's return and Cory Allen's arrival from Cardiff Blues, but domestic form hasn't been promising, as they prop up Conference A in the Guinness PRO14.
History in competition: Ospreys haven't progressed beyond the Pool stage since 2010 and three quarter-final appearances have been the limit to their European Cup achievements so far.
Key player: Alun Wyn Jones.
Opening three matches: Clermont (H), Saracens (A), Northampton (A).
Season overview: Billy Vunipola's injury is a blow for the defending champions, but they have made a fine addition in Liam Williams and the squad remains as ruthless and supremely confident as ever.
History in competition: Winners for the last two seasons and favourites to make that a hat-trick.
Key player: Liam Williams.
Opening three matches: Northampton (A), Ospreys (H), Clermont (H).
Season overview: Rob Baxter admits he has had to learn how to balance Europe with the Chiefs' domestic duties. Champions Cup success is definitely now the next target in the Premiership champions' remarkable rise.
History in competition: In their short Champions Cup history Exeter have made the knockout stages just once, falling at the first hurdle to Wasps in 2015/16.
Key player: Henry Slade.
Opening three matches: Glasgow (H), Montpellier (A), Leinster (H).
Season overview: Glasgow are the only unbeaten side in Europe's three major leagues, leading PRO14 Conference A by eight points after six games. Dave Rennie's men may land a few blows in what is likely to prove a taxing pool.
History in competition: Last year's quarter-final defeat to eventual winners Saracens was the first time Glasgow had got past the pool stage since 1998.
Key man: Finn Russell.
Opening three matches: Exeter (A), Leinster (H), Montpellier (H).
Season overview: Leinster haven't picked up silverware of any kind since 2013/14, but they are one of the traditional heavyweights at elite European level and a strong start in the PRO14 bodes well.
History in competition: Three-time winners (in 2009, 2011 and 2012) and nine-time semi-finalists, Leinster have serious top-level European pedigree.
Key man: Johnny Sexton.
Opening three matches: Montpellier (H), Glasgow (A), Exeter Chiefs (A).
Season overview: New coach Vern Cotter has an embarrassment of riches at his disposal, but can big-spending Montpellier gel in time to launch a serious tilt at Europe's top prize?
History in competition: Montpellier first qualified for the European Cup only six years ago, but have progressed past the pool stage just once in five attempts since.
Key man: Louis Picamoles.
Opening three matches: Leinster (A), Exeter (H), Glasgow (A).
Season overview: Castres perhaps lack some of the stardust of their French counterparts and will need to find an extra gear after an underwhelming start to their TOP 14 campaign.
History in competition: Castres are regular diners at Europe's top table, but they have reached the knockout stage just once in 13 attempts.
Key man: Alex Tulou.
Opening three matches: Munster (H), Leicester (A), Racing 92 (H).
Season overview: Leicester have lost their last two Aviva Premiership games and are pooled again with Munster and Racing 92, where they were beaten last season in what was a disappointing campaign.
History in competition: The Tigers have a rich pedigree. The first club to retain the European Cup in 2002, they have reached the final twice more since, but two Pool exits in the last three years have not pleased fans.
Key man: Matt Toomua.
Opening three matches: Racing 92 (A), Castres (H), Munster (A).
Season overview: Munster start favourites to top the pool, but their PRO14 defeat to Leinster last weekend was disappointing.
History in competition: Winners in 2006 and 2008 and losing finalists in 2000 and 2002, Munster have fallen in the semis in five of the last nine years, including last season, when they lost to champions Saracens.
Key man: Keith Earls.
Opening three matches: Castres (A), Racing 92 (H), Leicester (H).
Season overview: Racing struggled last season, having won the TOP 14 and reached the Champions Cup final the previous year, and they have made a modest start to this season.
History in competition: Racing first competed in Europe's elite competition seven years ago. Apart from a run to the final in 2016, they have surpassed the Pool stage just once.
Key man: Virimi Vakatawa.
Opening three matches: Leicester (H), Munster (A), Castres (A).
Season overview: Bath have made a promising start to the Aviva Premiership season, losing only two of their first six games to lie fifth, just four points behind the leaders.
History in competition: Bath were the first British club to win the European Cup in 1998, but the furthest they have progressed since was the semi-finals in 2006 and indeed this season return to the competition after a year in the Challenge Cup.
Key man: Semesa Rokoduguni.
Opening three matches: Benetton (H), Scarlets (A), Toulon (A).
Season overview: Benetton have hit form just in time for the Champions Cup, winning three of their last four games in the Guinness PRO14 to climb to fifth in Conference B.
History in competition: The Italian side are back in the Champions Cup after spending only their third campaign out of it last year, but they have a tough task to reach the knockout stage for the first time.
Key man: Jurie van Vuuren.
Opening three matches: Bath (A), Toulon (H), Scarlets (A).
Season overview: Last season's TOP 14 runners-up have endured a poor opening to their 2017-18 campaign, mixing four wins with three defeats to slip to sixth.
History in competition: Toulon became the first club to win the Champions Cup three years in a row in 2013, 2014 and 2015, but they have bowed out at the quarter-final stage the past two seasons.
Key man: Mathieu Bastareaud.
Opening three matches: Scarlets (H), Benetton (A), Bath (A).
Season overview: Scarlets lead Conference B in the Guinness PRO14 on bonus points after winning five of their six games and only losing in Ulster.
History in competition: The Welsh region have only once got beyond the quarter-finals in the Champions Cup – back in 2007 – and have exited after the Pool stage the last five seasons.
Key man: Gareth Davies.
Opening three matches: Toulon (A), Bath (H), Benetton (H).