The UEFA Champions League is an annual club football competition organised by the Union of European Football Associations and contested by top-division European clubs, deciding the competition winners through a group and knockout format. 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 of their national associations. Introduced in 1955 as the European Champion Clubs' Cup, commonly known as 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 and allowing multiple entrants from certain countries. It has since been expanded, and while most of Europe's national leagues can still only enter their champion, the strongest leagues now provide up to four teams.