League of Legendsではプレイヤーそれぞれが100以上いるチャンピオンから1人を選択（Pick）し、特定のルールの元戦闘を行う。選択できるチャンピオンは週替りの無料チャンピオンの他に、ゲーム内で溜まるポイントや課金ポイントを利用してアンロックすることで増やすことができる。アンロックに必要なポイントに差はあるが、高いからといって必ずしも強いわけではなく、プレイヤースキルとチームワークが勝敗を分けるため、安いチャンピオンでずっとプレイすることも問題ない。
As the League of Legends was established to prevent further Rune Wars on Valoran, the neutral aligned summoners called forth exceptional individuals, referred to as "Champions", to battle on the Fields of Justice in order to settle disputes. These champions have joined the League for various reasons. Some are soldiers who have pledged their allegiance to a particular nation and fight in their nation's interests. Some are renowned warriors who seek to prove themselves against Valoran's most powerful beings. Some are prisoners of the League, forced to do the League's bidding as punishment. Some may even be strange creatures from other worlds, hoping to bring stability or further discord to Runeterra. For whatever the champion's reason, they have gained the interest of the League of Legends.
In order to join the League, a potential champion, who has proven to possess extraordinary qualities, must travel to the Institute of War to undergo a Judgement. There, a designated summoner will probe their mind in order to determine the individual's motives. In truth, the intentions of the potential champion has little bearing on whether they are accepted into the League or not. The primary purpose of the Judgement is to determine whether the individual is capable of exposing their mind to the summoner, which would allow the two to share their mental thoughts and act as one. Should the potential champion pass their Judgement, as well as agreeing to the call of the summon, they will have become an official League champion.