The UEFA Champions League is an annual club football competition organized by the Union of European Football Associations (UEFA) and contested by top-tier European clubs. The competition is decided by a round-robin group stage, which qualifies teams for a double-leg knockout format, and a single leg final. It is one of the most prominent football events in the world, as well as the most prestigious club competition in European football, and it is contested by the champions of their respective national leagues.

The European Cup, formerly known as the Coupe des Clubs Winners Européens, was first staged in 1955 as a single-elimination tournament open only to the champions of Europe's domestic leagues, with the winner crowned European club champion. After adding a round-robin groupstage in 1991 and allowing multiple entries from some countries in 1997, the competition was renamed in 1992.

While most European national leagues can still only enter their champion, the most powerful leagues can now enter up to four teams. Teams that finish second to last in their national league after failing to qualify for the Champions League will be eligible for the second-tier UEFA Europa League competition, and teams that do not qualify for the UEFA Europa League will be eligible for the new third-tier UEFA Europa Conference League, which will begin in 2021.