Qualifications

As of 2011, the UEFA Champions League commences with a double round-robin group stage of 32 teams, which is preceded by two qualification 'streams' for teams that do not receive direct entry to the tournament proper. The two streams are divided between teams qualified by virtue of being league champions, and those qualified by virtue of finishing 2nd–4th in their national championship.
The number of teams that each association enters into the UEFA Champions League is based upon the UEFA coefficients of the member associations. These coefficients are generated by the results of clubs representing each association during the previous five Champions League and UEFA Europa League/UEFA Cup seasons. The higher an association's coefficient, the more teams represent the association in the Champions League, and the fewer qualification rounds the association's teams must compete in.
Five of the remaining ten qualifying places are granted to the winners of a four round qualifying tournament between the remaining 39 or 38 national champions, within which those champions from associations with higher coefficients receive byes to later rounds. The other five are granted to the winners of a two round qualifying tournament between the 15 clubs from the associations ranked 1 through 15, which have qualified based upon finishing second, third, or fourth in their respective national league.
In addition to sporting criteria, any club must be licensed by its national association to participate in the Champions league. To obtain a license, the club must meet certain stadium, infrastructure, and finance requirements.
In 2005–06, Liverpool and Artmedia Bratislava became the first teams to reach the Champions League group stage after playing in all three qualifying rounds. In 2008–09, both BATE Borisov and Anorthosis Famagusta achieved the same feat. Manchester United is the team that has appeared most often in the group stage, 18 times.
Between 2003 and 2008, no differentiation was made between champions and non-champions in qualification. The 16 top ranked teams spread across the biggest domestic leagues qualified directly for the tournament group stage. Prior to this, three preliminary knockout qualifying rounds whittled down the remaining teams, with different teams starting in different rounds.
An exception to the usual European qualification system happened in 2005, after Liverpool won the Champions League the year before, but did not finish in a Champions League qualification place in the Premier League that season. UEFA gave special dispensation for Liverpool to enter the Champions League, giving England five qualifiers. UEFA subsequently ruled that the defending champions qualify for the competition the following year regardless of their domestic league placing. However, for those leagues with four entrants in the Champions League, this means that if the Champions League winner falls outside of its domestic league's top four, it will qualify at the expense of the fourth-placed team in the league. No association can have more than four entrants in the Champions League. In May 2012, Tottenham Hotspur finished fourth in the Premier League, two places ahead of Chelsea, but failed to qualify for the 2012–13 Champions League, after Chelsea won the 2012 Champions League Final. Tottenham were demoted to the Europa League for the 2012–13 season.
The top three leagues in Europe are currently allowed to enter four teams into the Champions League. Michel Platini, the UEFA president, had proposed taking one place from the top three leagues and allocating it to that nation's cup winners. This proposal was rejected in a vote at a UEFA Strategy Council meeting. In the same meeting, however, it was agreed that the third-placed team in the top four leagues would receive automatic qualification for the group stage, rather than entry into the third qualifying round, while the fourth-placed team would enter the play-off round for non-champions, guaranteeing an opponent from one of the top 15 leagues in Europe. This was part of Platini's plan to increase the number of teams qualifying directly into the group stage, while simultaneously increasing the number of teams from lower-ranked nations in the group stage.