The new campaign marks the arrival of a revamped league-style format, with 36 teams competing in European soccer's premier club competition for the first time.
Here are the main takeaways from Tuesday's matches.
The Vincent Kompany era at Bayern Munich is already off to a flying start.
The German giant has won all three of its games in the Bundesliga and on Tuesday began its European campaign in emphatic fashion.
Bayern beat Dinamo Zagreb of Croatia 9-2 at Allianz Arena, becoming the first team in Champions League history to score nine goals in a single match. The last time Bayern achieved a similar feat in European soccer competition was in the 1983/84 UEFA Cup, beating Anorthosis Famagusta 10-0.
Striker Harry Kane scored four times, three of which came from the penalty spot, while Michael Olise got two on his debut in the competition - the first Frenchman since Thierry Henry in 1997 to score twice on his first Champions League appearance. Raphaël Guerreiro, Leroy Sané and Leon Goretzka scored Bayern's other three goals.
The total of 11 goals between the two teams is the second highest in the history of the competition, after Borussia Dortmund's 8-4 win over Legia Warsaw in November 2016.
After the victory, Kompany - who was appointed as Bayern manager in the summer having been at the helm at English club Burnley last season when it was relegated from the Premier League - addressed his critics.
"I'll tell you something in a quick way just to prove a point," Kompany told reporters. "I'm born in Brussels, northern quarter in Brussels, my dad was a refugee who came from Congo to Belgium.
"What were my chances of setting foot even in the Premier League, playing in the Premier League, winning something as a player, playing for the national team? The odds were 0.000-what percent? And now I'm a coach. The question is do you just stop believing in yourself because of what other people say? Do you stop believing in what you can achieve because of what other people say?
"The mentality is to continue to go and, in the end, if you fail, you fail; if you succeed, you succeed, but you can become better every single time. Online you can always find stuff, so I really don't take it personally."
Kylian Mbappé opened his Champions League account for Real, scoring just after half-time before Deniz Undav equalized for the German side.
But two late goals from Antonio Rüdiger and the 18-year-old Endrick sealed the victory for Real.
Endrick's goal came with extra significance, too, as he became the youngest goalscorer ever for Real Madrid in European competitions. The Brazilian's goal at the age of 18 years and 58 days surpassed the previous record set by club legend Raúl of 18 years and 113 days in 1995.
Afterwards, Madrid manager Carlo Ancelotti praised Endrick's impact off the bench, saying that "he is able to do things that no-one can think of."
"He has the gift that strikers dream of, the gift of being very effective, decisive," Ancelotti said. "You can see that he has something special, something I have never seen. And he has such a strong and very fast shooting.
"Endrick had courage because it was the last ball of the game. The best solution was to take advantage of the three against one, but he did it very well, even though it was perhaps the most complicated solution."
It was an opening night to remember for US participation in the Champions League, with two goals scored by American players across the six games.
The first came from Weston McKennie, who swept home the second goal in Juventus' comfortable 3-1 victory over PSV Eindhoven.
McKennie was one of five Americans on the pitch for both teams - Tim Weah lining up alongside McKennie for Juventus and PSV fielding Malik Tillman, Richard Ledezma and Ricardo Pepi. It was the first game in Champions League history to feature five US players, per Goal.com.
Later on Tuesday, US men's national team captain Christian Pulisic also got a goal, opening the scoring for AC Milan against Liverpool at the San Siro.
Pulisic's goal came in a losing effort for Milan, though, as goals from Ibrahima Konaté, Virgil van Dijk and Dominik Szoboszlai helped the Premier League giant mount a comeback to win 3-1.
It may only have been the opening night of Champions League action, but one of the goals of the competition might have already been scored.
It came in Sporting CP's clash with Lille in Lisbon via the right foot of Belgian defender Zeno Debast.
Having already been put ahead by Viktor Gyökeres' first-half strike, Sporting went in search of a second goal against 10-man Lille following Angel Gomes' red card.
And just after the hour mark, the home side doubled its lead in the most impressive fashion thanks to a thunderbolt strike from Debast.
The ball was laid back to the 20-year-old 30 yards from goal and, without taking a touch, he unleashed a powerful effort into the top corner. As he wheeled away in celebration, Debast's teammates were left with their hands on their heads in awe of the goal.
The-CNN-Wire
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