The most important club competition in Europe and is worth loads of money to teams that do well. It used to be called the European Cup and then only teams who won their own league could enter.
Now up to four teams are allowed to enter from the biggest leagues, England, Italy and Spain, with others having between one and three teams.
The biggest teams go straight into the first proper round of the tournament, where 32 teams play in eight groups of four. Some smaller clubs have to qualify to get into the groups.
The top two teams in each group go through to the second round of 16, and from then on the tournament changes to a knock-out format, with each pairing playing home and away.
The final is the last game of the European domestic season, near the end of May.