Match ends, Monaco 1, Atlético de Madrid 2.
Atletico Madrid came from behind to beat Monaco in their Champions League Group A opener.
Samuel Grandsir poked in an opener for the hosts after defensive mistakes from both Saul and Angel Correa.
Diego Costa converted Antoine Griezmann's excellent pass to level and Jose Gimenez headed in Koke's corner to give the visitors the lead.
But Atletico, with boss Diego Simeone in the stands, were almost pegged back late on when Kamil Glik headed wide.
The Argentine was serving the final game of a four-match European touchline ban dating back to last season's Europa League semi-final tie with Arsenal.
Winger Thomas Lemar, a £62m summer signing from Monaco, got a good reception when he came on in the second half for Atletico.
Atletico - whose Wanda Metropolitano ground hosts this season's final - are looking to avoid a repeat of last year when they went out in the group stages.
In the other game in Group A, Borussia Dortmund beat Club Brugge 1-0 thanks to Christian Pulisic's fortunate late winner on his 20th birthday. Dortmund's English teenager Jadon Sancho made his Champions League debut in the match.
In Group C, almost 50,000 fans watched as Red Star Belgrade drew 0-0 with Carlo Ancelotti's Napoli in their first Champions League group game since 1992.
Last year's finalists Liverpool beat Paris St-Germain 3-2 thanks to a late Roberto Firmino in the other Group C game.
FC Porto goalkeeper Iker Casillas became the first player to feature in 20 different Champions League campaigns as they drew 1-1 with Schalke in Group D. In the other game in that group, Galatasaray beat Lokomotiv Moscow 3-0.
In the early Group B kick-offs, Barcelona beat PSV Eindhoven 4-0 with a Lionel Messi hat-trick and Inter Milan scored twice in the last five minutes to beat Tottenham 2-1.
Match ends, Monaco 1, Atlético de Madrid 2.
Second Half ends, Monaco 1, Atlético de Madrid 2.
Antoine Griezmann (Atlético de Madrid) wins a free kick on the left wing.
Foul by Djibril Sidibe (Monaco).
Diego Costa (Atlético de Madrid) is shown the yellow card.
Offside, Atlético de Madrid. Lucas Hernández tries a through ball, but Diego Costa is caught offside.
Attempt missed. Kamil Glik (Monaco) header from the centre of the box is close, but misses to the left. Assisted by Jordi Mboula with a cross.
Rodrigo (Atlético de Madrid) wins a free kick on the left wing.
Dangerous play by Adama Traoré (Monaco).
Foul by Thomas Lemar (Atlético de Madrid).
Adama Traoré (Monaco) wins a free kick in the attacking half.
Attempt missed. Djibril Sidibe (Monaco) right footed shot from outside the box is high and wide to the left. Assisted by Kevin N'Doram.
Foul by Saúl Ñíguez (Atlético de Madrid).
Youri Tielemans (Monaco) wins a free kick on the right wing.
Adama Traoré (Monaco) is shown the yellow card for a bad foul.
Lucas Hernández (Atlético de Madrid) wins a free kick in the defensive half.
Foul by Adama Traoré (Monaco).
Substitution, Monaco. Moussa Sylla replaces Samuel Grandsir.
Corner, Atlético de Madrid. Conceded by Kamil Glik.
Corner, Atlético de Madrid. Conceded by Jemerson.
Foul by Diego Costa (Atlético de Madrid).
Djibril Sidibe (Monaco) wins a free kick in the defensive half.
Attempt blocked. Jordi Mboula (Monaco) left footed shot from the right side of the box is blocked. Assisted by Youri Tielemans.
Substitution, Atlético de Madrid. Thomas Lemar replaces Ángel Correa.
Substitution, Monaco. Adama Traoré replaces Jean Eudes Aholou.
Foul by Saúl Ñíguez (Atlético de Madrid).
Jordi Mboula (Monaco) wins a free kick in the defensive half.
Attempt missed. Radamel Falcao (Monaco) right footed shot from outside the box is high and wide to the left. Assisted by Kamil Glik.
Corner, Monaco. Conceded by Diego Costa.
Corner, Monaco. Conceded by Diego Godín.
Foul by Antoine Griezmann (Atlético de Madrid).
Kevin N'Doram (Monaco) wins a free kick in the defensive half.
Corner, Atlético de Madrid. Conceded by Djibril Sidibe.
Substitution, Monaco. Jordi Mboula replaces Nacer Chadli.
Attempt missed. Rodrigo (Atlético de Madrid) header from the left side of the six yard box is close, but misses to the left. Assisted by Antoine Griezmann with a cross following a corner.
Corner, Atlético de Madrid. Conceded by Youri Tielemans.
Djibril Sidibe (Monaco) is shown the yellow card for a bad foul.
Diego Costa (Atlético de Madrid) wins a free kick on the left wing.
Foul by Djibril Sidibe (Monaco).
Offside, Atlético de Madrid. Diego Godín tries a through ball, but Diego Costa is caught offside.