Six pools of four teams play both home and away games. Until the 2007/2008 season these pools were drawn mostly at random, with the following restrictions:
* Each nation nominates one of their teams as top seed; these teams are drawn in separate pools.
* Each nation supplies at most one team to each pool, except where England or France supply seven teams in total; in this case, the seventh team drawn will appear in a pool with one other team from that nation. In some cases (such as for the 2007/2008 season) the unseeded Italian and Scottish teams may also deliberately be drawn in different pools.
From the 08/09 season, there is more structure to the pools. The competing 24 teams are ranked based on past performance and arranged into four tiers of six teams, with the reigning champion automatically appearing in the top tier. Each pool receives one team at random from each tier; again, this is subject to the restriction that each pool cannot contain more than one team from each competing nation, except where France or England supply seven teams.
Four points are awarded for a win and two points for a draw. A bonus point is awarded for a loss by seven points or fewer, or for scoring four tries or more. The six pool winners (ranked 1-6 by number of points scored) and two best placed runners-up (ranked seven and eight) qualify for the quarter-finals. Teams ranked one to four have home advantage.