The UEFA Champions League comprises three qualifying rounds, a play-off round, a group stage and four knockout rounds.
In matches in the three qualifying rounds and the play-off stage, clubs play two matches against each other on a home-and-away basis. The club which scores the greater aggregate of goals qualifies for the next round, with away goals and then penalties used to determine the winner in the event of a draw.
The ten winners in the play-off round ties – five from the best-placed path and five from the champions path – join 22 automatic entrants in the 32-team group stage. The clubs are split into eight groups of four teams, who play home and away against each of their pool opponents between September and December to decide which two teams from each section advance to the first knockout round. The third-place finishers in each group enter the UEFA Europa League round of 32.
From the last 16 until the semi-finals, clubs play two matches against each other on a home-and-away basis with the same rules as the qualifying and play-off rounds applied. In the last 16, group winners play runners-up other than teams from their own pool or nation, while from the quarter-finals on the draw is free.
The final is decided by a single match, which this season will be played at Wembley Stadium on Saturday 25 May 2013.
Further details, including the criteria for separating teams that finish level on points in a group, can be found in the official competition regulations.