Manchester United and Arsenal both scored big wins in the Champions League, but Celtic lost their match.
United were 3-0 winners away to Aalborg in Denmark, with £30m striker Dimitar Berbatov scoring his first two goals for his new team.
Arsenal cruised to a 4-0 win over Porto at home, days after their shock defeat to Hull in the Premier League.
But Celtic lost 1-0 away to Villareal in Spain, and could find it difficult to qualify for the next round.
Marcos Senna bagged the goal for the Spanish side from a free-kick, while Georgios Samaras had the best chance for Celtic.
The Scottish champions are in the same group as Manchester United, and the two sides meet next in the competition.
Arsenal celebrate one of their four goals against Porto
And it could be a tough match for Celtic, as United look like they're starting to hit top form.
Wayne Rooney was the other goal scorer in Denmark, and Cristiano Ronaldo went close to scoring a number of times.
Gunners get going
United now have a good chance of getting out of the group stage, and Arsenal also look strong.
Robin van Persie and Emmanuel Adebayor each scored twice for the Gunners, who top their group after two matches.