Kill

A kill, is the event of decreasing an enemy champion's health to zero while the enemy has no abilities or items to prevent death. Death may be prevented by certain effects, such as those granted by the item and friendly abilities like, or delayed by passive abilities like  and.

Rewards
The last player to strike the target champion is awarded with the kill, and all contributors within the last 10 seconds are awarded with an assist. If the champion killed has any transferable buffs ( or ), the killer champion gains all of these buffs and the duration of each buff is reset. When a champion is killed, the player who scored the final hit receives gold (15-500g) and experience. To reduce the effectiveness of feeders and snowballing lanes, less gold is awarded when a champion has a lower level than the killers. Additionally, a champion with many consecutive deaths will award a significantly reduced bounty, eventually reaching a bounty floor of 15 gold, less than the gold value of a minion. The bounty will reset to 300 if said champion kills an enemy champion, or resets more gradually with assists. In gameplay, kills may be sacrificed in favor of securing a map objective, or due to other situational circumstances, such as the low health champion retreating into high risk enemy territory.

Execution
If a champion dies and hasn't been damaged or debuffed by another champion in the last 15 seconds, they are executed. Otherwise, the kill is awarded to the enemy champion who last dealt damage. Executions are caused by a turret, minion, or monster. An execution offers no gold but still grants experience to nearby champions on the opposite team.

In some cases it could be strategically advantageous for a player to purposefully be executed as an alternative to dying from an enemy champion; in doing so, the enemy receives no gold.

Note that executions do not end your killing spree.

Multi-kill
The act of killing several champions within 10 seconds of each other. The following comments are shown: "Summoner" has slain "(slain name) Summoner" for a [...]!'
 * Double Kill (2)
 * Triple Kill (3)
 * Quadra Kill (4)
 * Penta Kill (5)
 * Legendary Kill (6+)

There is no bonus associated with multi-kills other than having the stat displayed in your match history after the game.

Ace
An ace is defined as killing the last living champion of the enemy team. Scoring an ace is far easier than scoring a penta kill, since the only requisite is to kill the last enemy champion, not all of them. There is no gold bonus associated with aces.

Gold reward
The standard gain per kill is 300 gold. Every kill by a champion increases its value (the reward gained for killing that champion) by 20%, up to a maximum of 500 gold. Each death of a champion reduces its value, by 13% for the first death and 30% for every death afterwards, to a minimum of 15 gold. The bounty will reset to 300 if said champion kills an enemy champion, or resets more gradually with assists. Assists remove one stage of the death penalty each (if a champion was on his third death (182 gold), an assist would raise the value to that of two deaths (260 gold)). Every kill a champion achieves without dying (called a killing spree) increases that champion's bounty to a maximum of 500 gold as stated before. The gold bounty is not reset when the champion is executed without any enemy champion contributing to it. Being on a killing spree only affects the gold gained upon your death. First Blood (the first kill of the match) gives an additional 100 gold (400 total) beyond the basic amount for the kill.

Bounty values
Note that gold values will continue to drop past the 4th death, eventually stopping at 15 gold. Also, with lower killing spree bounty cap and increased bounty gain, there is no additional gold for a shut down beyond the 4th kill.

Prior to V1.0.0.152 values: Note that gold values will continue to drop past the 9th death, eventually stopping at 15 gold.

Prior to V1.0.0.120 values:

Items benefiting from kills
+5% cooldown reduction
 * - Restores 12% of your max mana on kill or assist.
 * - +10% critical strike chance. Unique Passive - Avarice: Gain an additional +3 gold every 10 seconds. Unique Passive - Greed: Gain an additional 2 gold every kill.
 * - Gain an additional 1 physical damage and 0.2% life steal per champion or minion kill. Maximum of +30 damage and +6% life steal. Half of the bonuses are lost upon death.
 * - Unique Passive: +2 attack damage per level collects trophies when killing Champions, and gains bonus effects based on how many trophies he has. Kills and assists grant 1 trophy, and 1 trophy is lost on death.
 * 3 Trophies: +10 armor penetration
 * 6 Trophies: +25 movement speed
 * 9 Trophies: Rengar's leap gains 150 bonus range.
 * 14 Trophies: duration is increased by 3 seconds. Additionally, Rengar's next ability used after activating Thrill of the Hunt generates 1 bonus Ferocity.
 * - +15 ability power, +80 health, +3 mana regen per 5 seconds. Passive - Restores 5 mana when you kill an enemy unit.
 * - Your champion gains 8 ability power per stack, receiving 2 stacks for a champion kill or 1 stack for an assist. This effect can stack 20 times; you lose a third of your stacks if you die. At 20 stacks, your champion's cooldowns are reduced by 15%.
 * - +45%  attack speed, Active: You gain 100%  attack speed and 100% critical strike for 3 seconds or 3 critical strikes - 60 second cooldown. Passive: This item does not grant any attack speed while on cooldown. Champion kills reduce the current cooldown by 50%.
 * - Your champion gains +5 damage per stack, receiving 2 stacks for a champion kill or 1 stack for an assist. This effect can stack 20 times; you lose a third of your stacks if you die. At 20 stacks, your champion's movement speed is increased 15%

Skills benefiting from kills

 * (Active): Akali moves through the shadows to quickly strike her target and deal magic damage. Akali gains an Essence of Shadow once every few seconds up to a total of 3, affected by cooldown reduction. The time to gain a charge doesn't count when at maximum charges. Additionally, Akali gains an Essence of Shadow for a kill or an assist.
 * (Active):  Alistar lets out a commanding war cry, instantly restoring health to himself, and nearby friendly units for half of that amount. The cooldown is reduced by 2 seconds each time a nearby enemy unit dies.
 * (Active): Annie shoots a mana infused fireball, dealing magic damage to her target. The mana cost is refunded if it kills the target.
 * (Passive): Whenever Cho'Gath kills a unit, he recovers health and mana. The values restored increase with Cho'Gath's level.
 * (Active): Target enemy takes true damage. If Feast kills the target, Cho'Gath grows larger and gains extra health and basic attack range, stacking up to six times. Cho'Gath loses half of these stacks, rounded up (Lost stacks being rounded up, thus, kept stacks rounded down), whenever he dies.
 * (Active): Darius leaps to target enemy champion and strikes a lethal blow, dealing true damage. For each stack of Hemorrhage on the target, Noxian Guillotine deals an additional 20% damage. Darius may recast Noxian Guillotine within 12 seconds if it kills the target.
 *  (Active): Evelynn removes all slows affecting her, gains bonus movement speed for 3 seconds and ignores unit collision. Champion kills and assists refresh Dark Frenzy's cooldown.
 * (Active): Fiora gains additional attack speed for 3 seconds. Each autoattack or Lunge she lands during this time increases her movement speed for 3 seconds, stacking up to 3 times. Killing a champion refreshes the cooldown on Burst of Speed. Assists reduce the cooldown by half of the base amount.
 * (Active): Gangplank shoots a target enemy with his pistol, dealing physical damage. If it is killed with this ability, he gains extra gold and refunds half the mana cost. This ability can critically strike and applies on-hit effects, including his passive.
 * (Active): Irelia dashes forward to strike her target, dealing physical damage. This applies on-hit effects. If it kills the target, Bladesurge's cooldown refreshes and refunds 35 mana.
 * (Innate passive): After a kill or an assist, Katarina's cooldowns are reduced by 15 seconds.
 * (Toggle Off): When Karthus kills a unit, he restores mana.
 * (Active): Kha'Zix leaps to an area, dealing physical damage to enemies in the area. Kills and assists refreshes Leap's cooldown.
 * (Active): Malzahar infects his target's mind, dealing magic damage every half seconds for 4 seconds. If the target dies while afflicted by the visions, they pass the curse on to a nearby enemy unit, refreshing the duration and restoring mana to Malzahar.
 * (Active): Increases Master Yi's movement speed by 40% and attack speed for a few seconds, as well as making him immune to any slow effects. Additionally, killing a champion refreshes all of Master Yi's Abilities. Assist refreshes all abilities by half.
 * (Active): Mordekaiser deals damage to the target and curses it for 10 seconds, stealing life. Damage is equal to a percentage of the target's maximum health. If the target dies while under the effect of the curse, their soul is enslaved and will follow Mordekaiser for 30 seconds. During this time, Mordekaiser gains 20% of their ability power and attack damage. The spirit gains 75% of Mordekaiser's adjusted damage, ability power, and health totals.
 * (Active): Nasus's next attack will deal additional damage. Siphoning Strike permanently gains 3 damage whenever it kills an enemy. The bonus is doubled to 6 stacked damage if the enemy killed by Siphoning Strike is a champion, large monster or large minion.
 * health bonus can be triggered by killing an enemy unit with any of Sion's damage sources, including autoattacks, champion abilities, summoner spells and item actives.
 * (Innate): Whenever Swain kills an enemy unit, he restores 9 (+1 per level) mana.
 * (Active): Tristana fires at the ground to propel herself to a target location, dealing magic damage and slowing surrounding units by 60% for 2.5 seconds when she lands. On kills or assists, Rocket Jump's cooldown resets.
 * (Innate passive): On champion kill or assist, Varus gains 40% attack speed for 6 seconds. On minion kill, Varus gains 20% attack speed for 3 seconds
 * (Passive): Veigar gains 1/2/3/4/5 bonus ability power for each champion kill from any damage source and 1 bonus ability power for each champion or minion kill with Baleful Strike. The bonus is doubled to 2 AP if the enemy killed by Baleful Strike is a champion, large monster or large minion.