Regeneration
A creature with Regeneration regains a constant amount of Hitpoints over time. The amount of Hitpoints regained is specified by the feature or ability that grants Regeneration. Creatures may have Regeneration at all times, may need to activate it as part of a power, or may gain Regeneration in response to certain factors (such as exposure to certain energy types).
Regeneration only allows a creature to regain lost Hitpoints and cannot elevate it above its Hitpoint Total. Hitpoints regained through Regeneration are gained at the end of the round. Regeneration also automatically removes any Ongoing Bleed and Blood Loss effects.
A creature which continually Regenerates can heal physical injuries over time. Every phase it automatically removes 1 box from all of its injuries that have the Physical keyword. Additionally, a physical lethal injury is healed (and the creature fully restored to life) after 1d4 hours (provided the Regeneration is not prevented in some way, such as dealing any damage of the type that prevents its Regeneration). The permanent effects of injuries are also removed an hour after the injury is healed. If an effect or Lethal Injury completely destroys a creature's body, the Regeneration no longer functions.
Regeneration removes 1 level of Blood Loss every phase (potentially restoring the creature to life if it died from Blood Loss).
Some damage types deal injuries that regeneration cannot heal (such as vile damage or annihilation damage).
Sometimes a creature's Regeneration is prevented, usually by the application of specific types of injuries (i.e. resulting from a particular damage type). In these cases, the creature is permanently killed if it takes a Lethal Injury of a type that prevents its Regeneration. If a creature's Regeneration cannot be prevented, it may be extremely difficult to permanently kill and require creative measures to eliminate.
Regeneration and Fast Healing are not cumulative; the greater of the two benefits applies. Additionally, if Regeneration is gained from multiple sources, only the greater effect applies.
Simple Monsters
Simple monsters with Regeneration automatically suffer a Lethal Injury when reduced to 0 Hitpoints. Unless the Lethal Injury was of a type that prevents its Regeneration, the Fatal aspect of the injury will heal and the creature will be restored to life in 1d4 hours. However, dealing any subsequent damage to a simple monster of the type that prevents its Regeneration, will prevent it from healing the lethal injury and kill the creature.