The UEFA Champions League (abbreviated as UCL) is an annual club association football competition organised by the Union of European Football Associations (UEFA) that is contested by top-division European clubs. The competition begins with a round robin league phase to qualify for the double-legged knockout rounds, and a single-leg final. It is the most-watched club competition in the world and the third most-watched football competition overall, behind only the UEFA European Championship and the FIFA World Cup. It is one of the most prestigious football tournaments in the world and the most prestigious club competition in European football, played by the national league champions (and, for some nations, one or more runners-up) of their national associations.

Introduced in 1955 as the Coupe des Clubs Champions Européens (French for European Champion Clubs' Cup), and commonly known as the European Cup, it was initially a straight knockout tournament open only to the champions of Europe's domestic leagues, with its winner reckoned as the European club champion. The competition took on its current name in 1992, adding a round-robin group stage in 1991 and allowing multiple entrants from certain countries since the 1997–98 season. While only the winners of many of Europe's national leagues can enter the competition, the top 5 leagues by coefficient provide four teams each by default, with a possibility for additional spots based on performance during the previous season. Clubs that finish below the qualifying spots are eligible for the second-tier UEFA Europa League competition, and since 2021, for the third-tier UEFA Conference League.

In its present format, the Champions League begins in early July with three qualifying rounds and a play-off round, all played over two legs. The seven surviving teams enter the league phase, joining 29 teams qualified in advance. The 36 teams each play eight opponents, four home and four away. The 24 highest-ranked teams proceed to the knockout phase that culminates with the final match in late May or early June. The winner of the Champions League automatically qualifies for the following year's Champions League, the UEFA Super Cup, and the FIFA Club World Cup.

Spanish clubs have the most victories (20 wins), followed by England (15 wins) and Italy (12 wins). England has the most winning teams, with six clubs having won the title. The competition has been won by 23 clubs and 13 of them have won it more than once. Real Madrid is the most successful club in the tournament's history, having won it 15 times. Madrid is the only club to have won it five times in a row (the first five editions). Only one club has won all of their matches in a single tournament en route to the tournament victory: Bayern Munich in the 2019–20 season. Real Madrid is the current European champion, having beaten Borussia Dortmund 2–0 in the 2024 final for their fifteenth title. The first British team to win the trophy was Celtic, winning it in 1967 over Inter Milan.