Matchday 4 of the 2013-14 UEFA Champions League concluded with eight matches Wednesday evening.
For the second matchday in a row, Borussia Dortmund and Arsenal competed in the marquee matchup, but there was drama across the continent in all eight games.
Keep reading for a recap of all the action.
Arsenal made a major statement of Champions League intent with a 1-0 victory at Borussia Dortmund in Group F.
The victory was a first for an English side at BVB's Westfalenstadion. The same teams met at Arsenal's Emirates Stadium last month, with Dortmund claiming a 2-1 victory with a late goal via a counter.
This time, however, Arsenal soaked up Dortmund's pressure before scoring the game's only goal against the run of play in the 62nd minute. Striker Olivier Giroud won a header in the box, directing the ball toward Ramsey, who nodded it past Roman Weidenfeller and into the net.
Dortmund, who were trying for an eighth straight home win in the Champions League, went close in the first half through defender Neven Subotic's volley, which rolled narrowly wide. Seven minutes before half-time, Henrikh Mkhitaryan poked his shot wide from Jakub Blaszczykowski's pass.
Arsenal keeper Wojciech Szczesny saved Marco Reus' header early in the second half. Reus then put the ball in the net, but not before being flagged for offside.
With the victory, Arsenal remained atop Group F, even on nine points with Napoli. Dortmund are now third, three points behind.
On target: 3-2
Lionel Messi scored twice as Barcelona clinched a spot in the knockout stage with a 3-1 victory at home to AC Milan in Group H.
Messi had not scored in any of his last three league matches. He broke his goal drought with a 30th minute penalty after AC Milan's Ignazio Abate was whistled for a foul on Neymar in the box. Replays showed the contact was light.
Sergio Busquets made it 2-0 with a glancing header in the 39th minute, but AC Milan pulled one back on the stroke of half-time. Kaka cut in from the left and played the ball across the face of goal. Barcelona defender Gerard Pique was in position, but could only turn the ball into his own net.
Messi then sealed the points in the 83rd minute, swapping passes with substitute Cesc Fabregas before beating Christian Abbiati with a chipped finish.
Italian striker Mario Balotelli started on the bench for AC Milan and entered the match at half-time, replacing the ineffective Robinho.
Barcelona lead Group H with 10 points. Milan are second with five points, one ahead of Ajax.
On target: 6-1
Striker Samuel Eto'o scored twice as Chelsea went three points clear in Group E with a comfortable 3-0 win at home to Schalke.
The visitors started well but found themselves behind in the 31st minute after a freak goal. Goalkeeper Timo Hildebrand attempted to play a long pass up the pitch, but Eto'o rushed forward to block the ball—which instead deflected into Schalke's goal for a 1-0 Chelsea lead.
Eto'o then made it 2-0 in the 54th minute, and Demba Ba wrapped up the points with Chelsea's third seven minutes from time.
Chelsea manager Jose Mourinho made six changes following Saturday's loss at Newcastle United in the Premier League. Eto'o himself was in the starting lineup in place of the injured Fernando Torres, who scored twice in Chelsea's 3-0 win at Schalke last month.
With the win, Chelsea moved clear in first place on nine points. Schalke are second with six points, one ahead of third-placed Basel.
On target: 7-2
FC Basel 1-1 Steaua Bucharest
Giovanni Sio scored a late equaliser in second-half stoppage time for Basel, who escaped with a draw at home to Steaua.
Steaua had taken the lead in the 17th minute through Federico Piovaccari.
Basel are third in Group E with five points. Steaua remain fourth with two points.
Napoli 3-2 Marseille
Napoli kept pace with Arsenal in Group F with a thrilling 3-2 win at home to Marseille.
Gonzalo Higuain scored twice, including the 75th-minute winner. Marseille took an early lead through Andre Ayew, only for Gokhan Inler to equalise with a sweet volley in the 22nd minute.
Higuain's first goal put Napoli ahead 2-1 in the 24th minute, but Florian Thauvin pegged back the hosts midway through the second half.
With the win, Napoli moved onto nine points, level with Arsenal atop the group. Marseille still have not claimed a point and have been eliminated from contention for the knockout phase.
Zenit St. Petersburg 1-1 Porto
Hulk scored a goal and missed a penalty as Zenit drew with Porto in Wednesday's early kickoff.
The Brazilian forward, who formerly played for Porto, canceled out Lucho Gonalez's opener with a 28th-minute strike. But his weak penalty was saved in the 52nd minute as Porto held on for a point.
Zenit are second in Group G with five points, one ahead of Porto.
Atletico Madrid 4-0 Austria Vienna
Atletico booked passage to the knockout stage with an easy 4-0 win at home to Austria Vienna in Group G.
Miranda, Raul Garcia, Filipe Luis and Diego Costa scored the goals for Atleti. Costa missed a penalty in the second half.
Atletico lead Group G with a maximum 12 points from four matches. Austria Vienna have one point.
Ajax 1-0 Celtic
Lasse Schone scored the game's only goal in the 51st minute as Ajax claimed their first win of the group stage.
The Dutch side are now third in Group H with four points, just one behind second-placed AC Milan. Celtic are fourth with three points, having beaten Ajax in the return fixture two weeks ago.