The UEFA Champions League is an annual continental club football competition organised by the Union of European Football Associations (UEFA) and contested by top-division European clubs. It is one of the most prestigious tournaments in the world and the most prestigious club competition in European football, played by the national league champion (and, for some nations, one or more runners-up) of each UEFA national association. The final of the 2012–13 tournament was the most watched UEFA Champions League final to date, as well as the most watched annual sporting event worldwide in 2013, drawing 360 million television viewers. Introduced in 1992, the competition replaced the European Champion Clubs' Cup, or simply European Cup, which had run since 1955, adding a group stage to the competition and allowing multiple entrants from certain countries. The pre-1992 competition was initially a straight knockout tournament open only to the champion club of each country. During the 1990s, the format was expanded, incorporating a round-robin group stage to include clubs that finished runner-up of some nations' top level league. While most of Europe's national leagues can still only enter their national league champion, Europe's strongest national leagues now provide up to four teams for the competition, and will provide up to five teams from the 2015–16 season onwards. Clubs that finish next-in-line in each nation's top level league, having not qualified for the UEFA Champions League competition, may be eligible for the next level UEFA Europa League competition.