Cooldown reduction

Cooldown reduction (abbreviated as CDR) reduces the cooldown of abilities, or the amount of time you must wait before an ability can be used again, once you activated it. Cooldowns cannot be reduced by more than 40%.

Cooldown reduction works additively. If you have a 6% cooldown reduction from masteries, and a 10% cooldown reduction from you will have a 16% cooldown reduction.

Cooldown reduction is unique among the same item. So 2 (+15%) will only give the bonus of +15%, but a  (+15%) and a  (+10%) will give a +25% bonus.

CDR is determined at the time an ability is used. Increasing CDR while the skill is on cooldown will not affect the cooldown.

and  are the only skills in the game that are not affected by CDR.

Items
There are several items which reduce the cooldown of your abilities. Cooldown Reduction granted by an item is always Unique Passive.
 * ': +15% Cooldown Reduction. +60 AP, +10 Mana Regen per 5, UNIQUE Active: Deals magic damage to target champion equal to 30% of their current health (+3.5% per 100 Ability Power) with a minimum of 200 damage. 1 minute cooldown. 2610 Gold.'
 * ': +10% Cooldown Reduction. +30 AP, +7 Mana Regeneration per 5. 1245 Gold.'
 * ': +20% Cooldown Reduction. +99 Armor, +500 mana, UNIQUE Passive: Reduces the attack speed of nearby enemies by 20%. 2775 Gold.'
 * ': +15% Cooldown Reduction. +425 Mana, +45 Armor. 1525 Gold.'
 * ': +15% Cooldown Reduction. UNIQUE Passive: Enhanced Movement 2. 1050 Gold.'
 * ': +10% Cooldown Reduction. +200 Health. 850 Gold.'
 * ': +15% Cooldown Reduction (at 20 stacks). +20 AP, UNIQUE Passive: Your champion gains 8 ability power per stack, receiving 2 stacks for a kill or 1 stack for an assist. This effect can stack 20 times; you lose a third of your stacks if you die. 1235 Gold.'
 * ': +20% Cooldown Reduction. +75 AP, +12 Mana Regen per 5. 2350 Gold.'
 * ': +25% Cooldown Reduction. +50% Attack Speed, +55 AP, +10 Mana Regeneration per 5. 2885 Gold.'
 * ': +5% Cooldown Reduction. +300 Health, +75 Armor, +25 Health Regen per 5 sec, UNIQUE Passive: 20% chance on being hit to slow the attacker's movement and attack speeds by 25% for 3 seconds, UNIQUE Active: Slows movement speeds and attack speeds of surrounding units by 35% for 1 second + 0.5 seconds for each 100 Armor and Magic Resist. 60 second cooldown. 3075 Gold.'
 * ': +15% Cooldown Reduction. +330 Health, +30 Health Regen per 5 sec, +15 Mana Regen per 5 sec, UNIQUE Active: Nearby champions gain 40% movement speed for 3 seconds (60 second cooldown). 2200 Gold.'
 * ': +10% Cooldown Reduction (to allied nearby champs). +520 Health, UNIQUE Aura: Gives nearby Champions 12 Mana Regen per 5. 2285 Gold.'
 * ': +10% Cooldown Reduction. +30 Magic Resistance, +250 Health, UNIQUE Passive: Increases healing and regeneration on yourself by 15%. 1550 Gold.'
 * ': +10% Cooldown Reduction. +40% Attack Speed. 1140 Gold.'
 * ': +10% Cooldown Reduction. +25 Attack Damage, UNIQUE Passive: +15 Armor Penetration. 1337 Gold.'
 * ': +15% Cooldown Reduction. +30 Attack Damage, +15% Critical Chance, UNIQUE Passive: +20 Armor Penetration, Unique Active: You gain +20% movespeed and +50% attack speed for 4 seconds. Attacking enemy units with melee attacks increases the duration by 2 to a maximum of 8 seconds. 60 second cooldown. 2687 Gold.'

Consumable

 * : +10% Cooldown Reduction +20-40 Ability Power based on champion level for 4 minutes.

Champion Abilities

 * increases Cooldown Reduction by 10/15/20%.
 * increases Cooldown Reduction by 2/4/6/8/10%.
 * increases Cooldown Reduction by 3/6/9/12/15%.

Masteries

 * increases Cooldown Reduction by 0.75/1.5/2.25/3%.
 * increases Cooldown Reduction by 2/4/6%.

Runes

 * Celerity and Focus runes increase Cooldown Reduction.

Neutral Buffs

 * The buff increases Cooldown Reduction by (16+level*0.5)%.
 * The buff increases Cooldown Reduction by 10%.

Lowering Cooldowns
Some champions have abilities that decrease their cooldowns by a flat amount or entirely refresh their cooldown. By using those, a champion can go beyond the 40% cooldown reduction cap.

Flat Reduction

 * cooldown is reduced by 2 seconds each time a nearby enemy unit dies.
 * cooldown is reduced by 0.5 seconds each time Amumu is hit by an attack.
 * 's cooldown is reduced to 0.5 seconds if the target is poisoned.
 * reduces all of Ezreal's cooldowns by 1 second if it strikes an enemy unit.
 * reduces all her cooldowns by 15 seconds whenever she gets a kill or an assist.
 * cooldown is reduced by 6 seconds if he manages to pick up his axe after using the ability.
 * allows him to reduce all of his other cooldowns by 1 when he uses an ability.
 * 's cooldown is reduced by 2 seconds with every basic attack.
 * reduce all of his other abilities' cooldowns by 1 second with every basic attack for 5 seconds.
 * reduces all of Zilean's other cooldowns by 10 seconds.

Cooldown Refreshing

 * refreshes all Master Yi's cooldowns whenever he kills a champion while under the effect of the skill. Additionally, his cooldowns are refreshed by half whenever he gets an assist.
 * cooldown is refreshed if it kills a unit.
 * cooldown is refreshed whenever she gets a kill or an assist.
 * cooldown is refreshed every time an enemy champion dies.