Health

Health (commonly known as HP, an abbreviation for Health/Hit Points) is the amount of life a unit or structure has. Max health is the cap on life any unit can have. Current health is directly reduced by damage and is regained by in different ways.

Every unit and structure has their max and current health displayed as a bar above their models. When displayed over a champion's model in game, the health meter is displayed as segmented bars.

Additionally, for your own champion, it is represented in the interface as a green bar with two numbers ( # / # ): the first one represents the actual life available at the moment, while the second is the maximum amount of health the champion can have. If a champion health reaches zero and he doesn't have any abilities preventing it, death will occur. Max health scales additively with every bonus health point and increases with each level, meaning that each bonus point acquired directly affects the statistic.

At level 18 base health ranges from 1610 health to  2381 health. See here for the complete list.

Health as a casting resource
A few champions also use their own health to cast abilities instead of mana:

Champion abilities

 * heals 40/55/70% of his maximum health over 12 seconds.
 * does 8% of his bonus health in additional magic damage.
 * increases his ability power by 1 for every 40 bonus health points.
 * does 15% of his bonus health in additional magic damage.
 * heals 30% of his maximum health over 6 seconds if he gets under 30% health.
 * does 18% of his bonus health in additional physical damage.
 * increases his attack damage by 1% of his maximum health.

Items

 * increases your attack damage by 2% of your maximum health.
 * does 30% (+3.5% per 100 ability power) of the target's current health in magic damage upon activation.
 * does 4% of the target's maximum health in magic damage every autoattack.

Ways to restore health
A champion's health can be restored in several ways:
 * Health regeneration
 * Using a.
 * Returning to the spawning pool, which restores a percentage of your maximum health per second.
 * Some max health is granted upon leveling up and with that you regain some current health. Actual health regained is lower depending on how wounded the champion is upon leveling up.
 * With life steal or spell vamp, dealing damage to enemies with autoattacks or abilities respectively will restore some health.
 * The passive effect of and  restores 250 health over 8 seconds when leveling up.
 * Using a healing ability or spell.

Core

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Champion abilities
Note: Only the addtional health effect of these abilities is shown here, to read more information on each of these abilities, follow the link on each of them.
 * increases his maximum health by 90 / 120 / 150 when it kills a unit. This buff stackas up to 6 times. Cho'Gath loses half his current stacks (rounded up) upon death.
 * passively increases his maximum health by 3 per bonus attack damage and by 2 per point of ability power.
 * increases his maximum health by 300 / 450 / 600 for 15 seconds.
 * increases his maximum health by 300 / 450 / 600 for 15 seconds.
 * increases his maximum health by 1 per 4 mana he has.
 * permanently increases his maximum health by 1 / 1.5 / 2 / 2.5 / 3 whenever he kills a unit if is toggled on.
 * increases his maximum health by 1.8 per 1 point of ability power.

Masteries

 * increases your champion's health by 1.5 per level (6 per level at rank 4). At rank 4 this translates to an additional 108 health at champion level 18.
 * increases your champion's health by 30.
 * increases your champion's maximum health by 3%.

Runes

 * and runes increase maximum health.