Key players, top signings, ones to watch: all you need to know about the 32 teams in the 2021/22 group stage.
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Holders Chelsea are among the 32 teams looking to make it all the way to the final in Saint Petersburg in this season's competition.
UEFA.com profiles all the contenders.
UEFA coefficient ranking (end of 2020/21): 53
How they qualified: Italian runners-up
Last season: UEFA Europa League round of 16 (L 1-2agg vs Man. United)
Best European Cup performance: winners (1962/63, 1968/69, 1988/89, 1989/90, 1993/94, 2002/03, 2006/07)
Coach: Stefano Pioli
The former defender has won over everybody at Milan with his calm and relaxed approach – not to mention good results, including qualification for the Champions League after a seven-year wait and a season in which the Rossoneri flirted with the Scudetto.
Key player: Zlatan Ibrahimović
At 39, the Swedish striker is still going strong. He not only scored 15 goals in 19 Serie A games last season but his presence and leadership undoubtedly helped the entire Rossoneri squad to improve. "You see Zlatan and you play better," said right-back Davide Calabria.
Big summer signing: Olivier Giroud
Milan struggled in games when Zlatan was not available last season. That should not happen now thanks to the presence of the experienced French international, who joined from Chelsea. Many, however, believe that the two strikers can also do great things together. The experience and talent are certainly there.
One to watch: Brahim Díaz
When Hakan Çalhanoğlu left Milan to join rivals Inter, the 22-year-old Spaniard immediately took the No10 shirt, a clear sign of his intention to have a more prominent role with the Rossoneri this season despite scoring seven goals in 39 matches in all competitions last season.
Did you know?
Despite being the second most successful team in the competition with seven triumphs, Milan have not played a game in the Champions League since 11 March 2014, when they lost 4-1 against Atlético.
UEFA coefficient ranking (end of 2020/21): 17
How they qualified: Dutch champions
Last season: group stage (UEFA Europa League quarter-finals)
Best European Cup performance: winners (1970/71, 1971/72, 1972/73, 1994/95)
Coach: Erik ten Hag
Amid speculation about his future, the much-coveted 51-year-old extended his contract by a year last season. Having already turned Ajax into a powerhouse in the Netherlands, Ten Hag will be keen to replicate the performances in Europe that took his side agonisingly close to reaching the 2019 UEFA Champions League final.
Key player: Dušan Tadić
The Serbian's Ajax statistics are phenomenal: 78 goals and 71 assists in 153 matches. Playing wide on the left or as a false nine, he is extraordinarily adept at holding up the ball and creating space for team-mates to exploit, while his finishing with both feet is generally exquisite.
Big summer signing: Steven Berghuis
The Dutch international winger made the unusual step of joining straight from Ajax's rivals Feyenoord, but the promise of trophies and UEFA Champions League action was too strong to resist. His superb crossing, dead-ball deliveries and unerring eye for goal will bring even greater menace to the Ajax attack.
One to watch: Ryan Gravenberch
The 19-year-old became a vital cog in the Ajax midfield last season, playing in 47 Eredivisie, cup and European matches. He also picked up the first of his seven Dutch caps. This elegant, strong, smart and skilful product of the Ajax youth development system has everything required to reach the very top.
Did you know?
This was the first summer since 2015 that Ajax didn't sell a player from their own academy for at least €13m.
UEFA coefficient ranking (end of 2020/21): 27
How they qualified: third in Italy
Last season: round of 16 (L 1-4agg vs Real Madrid)
Best European Cup performance: quarter-finals (2019/20)
Coach: Gian Piero Gasperini
The 63-year-old tactician is the mastermind behind Atalanta's miracle results in recent years, including three consecutive third-place finishes in Serie A and the Champions League quarter-finals in 2020. "Facing Gasperini's Atalanta is like going to the dentist," says Manchester City coach Pep Guardiola.
Key player: Robin Gosens
Gosens (Germany) and Joakim Mæhle (Denmark) were among the most exciting wing-backs at this summer's EURO. With Atalanta using a three-man back line, wingers become crucial, and Gosens has proved he can do it all on that left flank.
Big summer signing: Merih Demiral
Replacing Cristian Romero, who joined Tottenham, is not an easy task, but Demiral has shown great potential in the first part of his career. Centre-backs usually improve drastically under Gasperini, and the Turkish international could benefit from this move.
One to watch: Matteo Lovato
The same thing can be said for Lovato. The Italy Under-21 international joined in the summer from Verona after a convincing season with the Scaligeri, which featured 24 appearances.
Did you know?
Of all the teams that have debuted in the UEFA Champions League since 2003/04, Atalanta are only the third to qualify for the knockout phase in both of their first two appearances – Villarreal and Sevilla are the others.
UEFA coefficient ranking (end of 2020/21): 6
How they qualified: Spanish champions
Last season: round of 16 (L 0-3agg vs Chelsea)
Best European Cup performance: runners-up (1973/74, 2013/14, 2015/16)
Coach: Diego Simeone
Approaching ten years at the helm, Simeone has coached Los Rojiblancos to two Europa League titles, two UEFA Super Cups and two Champions League finals. Last season, he added a second Liga title to show his powers remain undiminished.
Key player: Luis Suárez
Signed from Barcelona in summer 2020, the Uruguayan striker has proved an inspired acquisition. He plundered 21 league goals in 2020/21, the last of them ensuring his club would claim their first title for seven years.
Big summer signing: Antoine Griezmann
Look who's back. The France striker may have to win over the fans again after his departure to Barcelona two year ago, but he returns as the fifth most prolific scorer in Atleti history, having struck 133 goals in 257 games between 2014 and 2019. "This is the best place for me to be happy on and off the pitch," he says.
One to watch: João Félix
Still only 21, there is surely more to come from the club's record signing. Nevertheless, he still made it into double figure for goals last season, with his ability and industry endearing him to Simeone.
Did you know?
Last season, Koke became the first player to make 100 appearances in European competition for Atlético.
UEFA coefficient ranking (end of 2020/21): 4
How they qualified: third in Spain
Last season: round of 16 (L 2-5agg vs Paris)
Best European Cup performance: winners (1991/92, 2005/06, 2008/09, 2010/11, 2014/15)
Coach: Ronald Koeman
A Camp Nou favourite as a player, Koeman is overseeing a changing of the guard at Barça at the start of the club's post-Lionel Messi era. His debut season brought victory in the Copa del Rey and the club will want more silverware this term to show life goes on.
Key player: Frenkie de Jong
A key figure since joining from Ajax in 2019, De Jong has slotted in seamlessly in the middle of the park. His crisp passing and clever use of the ball mean he looks every inch the classic Barcelona midfielder, while his tireless running and work ethic make him invaluable for Koeman.
Big summer signing: Memphis Depay
Barcelona fans need a new hero and in Depay it looks like they may have found their man. Not lacking in confidence, the Dutchman has has quickly established himself as central to the team's attacking play.
One to watch: Pedri
The midfielder has been a revelation for club and country over the past 12 months. Former Barça and current Spain coach Luis Enrique says that what Pedri can do "has never been done before by an 18-year-old, not even by Andrés Iniesta".
Did you know?
Barcelona's last Champions League game before Messi appeared for the club in Europe was a 1-1 home draw with Celtic in November 2004.
UEFA coefficient ranking (end of 2020/21): 1
How they qualified: German champions
Last season: quarter-finals (L on away goals vs Paris)
Best European Cup performance: winners (1973/74, 1974/75, 1975/76, 2000/01, 2012/13, 2019/20)
Coach: Julian Nagelsmann
The 34-year-old has big shoes to fill after Hansi Flick's reign. In 2016, he became the youngest coach in Bundesliga history when he took the Hoffenheim reins aged 28, leading them into the Champions League by finishing fourth the following season. He continued to impress in two seasons at Leipzig prior to joining Bayern in the summer.
Key player: Robert Lewandowski
The relentless Polish marksman hit new highs last term, scoring a record 41 German league goals as Bayern sealed a ninth consecutive title. He also rose to third on the list of all-time scorers in the Champions League by overhauling Raúl González.
Big summer signing: Dayot Upamecano
The 22-year-old centre-back went from strength to strength after joining Leipzig from Salzburg in 2017, impressing under Nagelsmann in the Champions League in recent seasons. The strong, pacy defender fills the void left by the departed David Alaba and Jérôme Boateng.
One to watch: Jamal Musiala
Became the youngest player to represent Bayern in the Bundesliga, aged 17 years and 115 days, in June 2020 and three months later broke the record for the club's youngest goalscorer too. Ended last season with seven goals from 37 appearances.
Did you know?
Bayern established a new landmark Champions League winning streak of 15 games last season before drawing at Atlético on Matchday 5.
UEFA coefficient ranking (end of 2020/21): 24
How they qualified: play-offs (W 2-1agg vs PSV Eindhoven)
Last season: third qualifying round (L 1-2 vs PAOK)
Best European Cup performance: winners (1960/61, 1961/62)
Coach: Jorge Jesus
The 67-year-old faces the challenge of getting the Eagles back on track after they failed to pick up any silverware in the first year of his second spell in charge. During his first stint, from 2009 to 2015, Jorge Jesus became the most successful coach in the club's history by winning ten trophies: three League titles, a Portuguese Cup, a Super Cup and five League Cups.
Key player: João Mário
The 28-year-old won the Portuguese Liga last term with Sporting CP while on loan from Inter. He only arrived at Benfica this summer but has made an instant impact with his organisational skills, vision and ability to dictate the pace of the game. "He adds class to the team," says Jesus of the midfielder, a UEFA EURO 2016 winner with Portugal.
Big summer signing: Roman Yaremchuk
Groomed at Dynamo Kyiv but signed from Gent, the 1.91m-tall forward will be looking to impress on his debut in the UEFA Champions League proper. The 25-year-old helped Ukraine to reach the quarter-finals of UEFA EURO 2020 with two goals and one assist in five matches.
One to watch: Gonçalo Ramos
The Benfica academy graduate made his first-team debut in July 2020 as a late substitute in a 4-0 win at CD Aves, and marked it with two goals in six minutes. A quick forward who can also operate as an attacking midfielder, the 20-year-old boasts an eye for goal and has shone at youth level for both the Eagles and Portugal.
Did you know?
The Lisbon giants have reached seven European Cup finals, but have lost their last five (most recently against Milan in 1990) after winning the competition in 1961 and 1962.
UEFA coefficient ranking (end of 2020/21): 30
How they qualified: Turkish champions
Last season: UEFA Europa League third qualifying round (L on penalties vs Rio Ave)
Best European Cup performance: quarter-finals (1986/87)
Coach: Sergen Yalçın
The first player to turn out for all of Turkey's big four – Beşiktaş then Fenerbahçe, Galatasaray and Trabzonspor – the midfielder moved into coaching in the Black Eagles' youth ranks and returned after a ten-year absence to take the top job in 2020, winning a domestic double in his first season.
Key player: Rachid Ghezzal
The French-born Algerian international winger had a great loan spell last term, scoring eight goals and setting up 17 more. Beşiktaş needed to put in a huge effort to sign him from Leicester on a permanent basis, with city rivals Galatasaray taking a close interest.
Big summer signing: Valentin Rosier
The signing of Brazilian forward Alex Teixeira was a coup, yet fans also swarmed to the airport this summer to greet Frenchman Rosier when he rejoined for a second season after being loaned last term by Sporting CP. Plenty of big clubs had been alerted to his skills and assists during Beşiktaş's double success of 2020/21.
One to watch: Ersin Destanoğlu
Beşiktaş have made a major goalkeeping reinforcement with the addition of the experienced Mert Günök; however, youth-team product Ersin has kept his place in the starting line-up, albeit amid heightened interest in the 20-year-old from foreign clubs.
Did you know?
Beşiktaş have fewer fans than Galatasaray and Fenerbahçe (their capital rivals are backed by an estimated 33% and 29% of the Turkish population), but their followers are seen as being the most passionate and the loudest. A cheer of 132 decibels was recorded in the wake of their 2-1 Champions League win against Liverpool on 24 October 2007 – approximately equivalent to the sound of a military jet taking off.
UEFA coefficient ranking (end of 2020/21): 11
How they qualified: UEFA Champions League winners
Last season: winners (W 1-0 vs Man. City)
Best European Cup performance: winners (2011/12, 2020/21)
Coach: Thomas Tuchel
Took over in January and immed