12 teams are directly qualified for the preliminary round (TH: Champions League title holders).

The draw for the group stage was held on 5 September 2018 in Barcelona.

In each group, teams play against each other home-and-away in a round-robin format.

The group winners, runners-ups and third placed teams advance to the Final 8.

The matchdays are 17 October, 3 and 21 November, 5 and 19 December 2018, 9 and 23 January, 6 and 27 February, 16 and 27 March, 13 and 24 April, 15 May 2019.