Bayern Munich won the Champions League last season while Sevilla claimed the Europa League - but there is little time to rest on their successes.
Teams across the continent will discover their European fate when the Champions League draw takes place on Thursday (16:00 BST) and the Europa League draw on Friday (12:00 BST).
Here is everything you need to know.
Champions League How does the draw work? Teams will be split into four pots, with pot one containing the holders, the Europa League winners and champions of the six highest-ranked nations.
Pots two, three and four are determined by Uefa's club coefficient rankings.
There are 26 teams who gained direct entry to the group stage and they will be joined by the six winners of the play-off ties.
No team can be draw against another from their own association, meaning the four English sides Liverpool, Manchester City, Manchester United and Chelsea will be kept apart.
Any changes to be aware of? Last season's final took place on 23 August which means the group stages start later than usual.
However, the knockout ties scheduled for March, April and May remain unchanged, with the final taking place on 29 May at Istanbul's Ataturk Stadium.
Matches will take place on a home and away basis but it remains to be seen whether fans will be allowed to attend.
What is the British interest? Liverpool are in pot one after winning the Premier League last season, while runners-up Manchester City, third-placed Manchester United and Chelsea who finished fourth are all in pot two.
Scottish champions Celtic missed out on a place in the competition after suffering a shock defeat against Ferencvaros in the second qualifying round.
What are the pots? Pot one: Bayern Munich, Sevilla, Real Madrid, Liverpool, Juventus, Paris St-Germain, Zenit St Petersburg, Porto.
Pot two: Barcelona, Atletico Madrid, Manchester City, Manchester United, Shakhtar Donetsk, Borussia Dortmund, Chelsea, Ajax.
Pots three or four: RB Leipzig, Inter Milan, Lazio, Atalanta, Lokomotiv Moscow, Marseille, Club Brugge, Borussia Monchengladbach, Istanbul Basaksehir, Rennes.
*Six more teams from the play-offs will be added to this group before the pots are split in two When are the match days? Matchday one: 20/21 October Matchday two: 27/28 October Matchday three: 3/4 November Matchday four: 24/25 November Matchday five: 1/2 December Matchday six: 8/9 December.