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. Adding Cooldown Reduction while waiting will not affect the countdown until the next time it's used.

Items that reduce cooldown
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.'
 * ': +25% Cooldown Reduction. +99 Armor, +500 mana, UNIQUE Passive: Reduces the attack speed of nearby enemies by 20%. 3075 Gold.'
 * ': +20% Cooldown Reduction. +425 Mana, +45 Armor. 1675 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.'
 * ': +25% Cooldown Reduction. +50% Attack Speed, +55 AP, +10 Mana Regeneration per 5. 2885 Gold.'
 * ': +8% Cooldown Reduction. +300 Health, +80 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. 2925 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.'
 * ': +15% Cooldown Reduction (to allied nearby champs). +520 Health, UNIQUE Aura: Gives nearby Champions 12 Mana per 5. 2285 Gold.'
 * ': +12% Cooldown Reduction. +30 Magic Resistance, +250 Health, UNIQUE Passive: Increases healing and regeneration on yourself by 20%. 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.'
 * + CDR Boots

Consumable

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

Other Ways to reduce Cooldowns

 * Neutral minion buffs such as:
 * Crest of Nature's Fury, granted by Rabid Wolf on Twisted Treeline: +10% Cooldown Reduction and +20% Attack Speed.
 * Crest of the Ancient Golem, granted by Ancient Golem on Summoner's Rift: +24% Cooldown Reduction and regenerate 1.5% of max mana or energy per second.
 * Sorcery (Offense Mastery): +0.75% Cooldown Reduction per Rank (max. +3% at Rank 4).
 * Intelligence (Utility Mastery): +2% Cooldown Reduction per Rank (max. +6% at Rank 3).
 * Celerity Runes (see Rune List)
 * Focus Runes (see Rune List)
 * Some champions also have abilities which can decrease their cooldowns
 * - Ryze
 * - Zilean
 * - Twisted Fate
 * - Heimerdinger
 * Battle Cry - Xin Zhao