In each group, teams play against each other home-and-away in a round-robin format. The group winners and runners-up advance to the round of 16, while the third-placed teams enter the 2018–19 UEFA Europa League round of 32. The matchdays are 18–19 September, 2–3 October, 23–24 October, 6–7 November, 27–28 November, and 11–12 December 2018.