Teams that want to qualify for the highly-rated Champions League have to either qualify or meet certain standards set by the Union of European Football Association (UEFA).
The UEFA uses a coefficient system to decide how many teams from each country gain entry into the group stages and how many must go through Champions League qualifying.
The coefficient is affected by the performance of the clubs in the european competition of not just this season but also in previous seasons. As per the above image, the higher the coefficient the more teams from that league are eligible for the CL next season.
For Automatic Qualification;
The teams that occupy the top three league places in the countries ranked first through third in UEFA competition gain automatic entry into the group stages for the following season’s Champions League competition.
The first- and second-place teams in the countries ranked fourth through sixth also gain automatic entry, as do the champions in the countries ranked seventh through 12th.
:max_bytes(150000):strip_icc():format(webp)/afc-champions-league-2017-preliminary-stage-kitchee-sc-hkg-vs-hanoi-fc-vie-639969136-588fc7005f9b5874ee605e4c.jpg)
The Champions League holders automatically get the chance to defend their title in the following season’s competition.
For exceptional Cases;
Any team who wins the Europa League also automatically gets the privilege to play next season’s Champions League even if they don’t qualify through their league standing.
For instance, Manchester United qualified for this season’s Champions League after winning last season’s Europa League though they finished sixth in the Premier League.