UEFA has charged Cologne and Arsenal following crowd problems at their Europa League game at the Emirates Stadium on Thursday.
The match was delayed by an hour as thousands of Cologne fans outside the stadium prevented Arsenal supporters from getting to the turnstiles.
On Friday, UEFA announced that it had opened disciplinary proceedings.
It has brought four charges against Cologne -- crowd disturbances, setting off fireworks, throwing objects and acts of damage.
Arsenal face one charge of violating security regulations because of blocked stairways in the away section, UEFA said.
An estimated 20,000 Cologne fans travelled to the game despite Arsenal having only allocated around 3,000 tickets to away supporters.
Thousands managed to buy tickets meant for home fans, but the atmosphere was largely peaceful throughout the match, which Arsenal won 3-1.
Flares were lit in the away section on two occasions, but were quickly put out by police.
UEFA said its disciplinary committee would deal with the case next Thursday, with its rules stating that the host club is "responsible for order and security both inside and around the stadium before, during and after matches."
The rules also say the hosts are "liable for incidents of any kind... unless they can prove that they have not been negligent in any way in the organisation of the match."
However, the regulations state that each club can be punished for "inappropriate behaviour on the part of their supporters."
Earlier on Friday, Arsenal had issued a statement saying they were conducting an investigation.
"We worked in full consultation with police and UEFA officials on the night and in advance of the game and had taken extensive steps in advance to prevent tickets being sold to visiting supporters," the club said.
"The 3,000 tickets issued to Cologne fans was in line with competition rules, but it is clear many more visiting fans arrived, causing significant congestion and disturbance outside the stadium before kick-off.
"Many tickets were sold through touts, and this is very disappointing."