Twenty clubs from the three major European domestic and regional leagues competed in the Champions Cup.

Note: Placing shown in brackets, denotes standing at the end of the regular season for their respective leagues, with their end of season positioning shown through CH for Champions, RU for Runner-up, SF for losing Semi-finalist, QF for losing Quarter-finalist, and PO for the Pro14 7th place play-off winner.

For the purpose of creating the tiers, clubs are ranked based on their domestic league performances and on their qualification for the knockout phases of their championships.

[6] Teams in the same pool play each other twice, at home and away, in the group stage that begins on the weekend of 12–14 October 2018, and continues through to 18–20 January 2019.

[7] In the event of a tie between two or more teams, the following tie-breakers are used, as directed by EPCR: [8] [9] [10] [11] [12] The eight qualifiers are ranked according to their performance in the pool stage and compete in the quarter-finals which will be held on the weekend of 28–31 March 2019.

[13] Also new for this season was the EPCR's use of discretion to allow semi-finals to be played at a qualified club's home venue.