Basic Regeneration Use

• When a creature with Regeneration is killed -- either by another creature or by a spell or effect that causes damage -- its owner can choose whether to pay the specified mana cost to regenerate it. If regenerated, the creature is tapped, removed from combat, and has its health restored to maximum.
  
  

Situations

• A player should use a Regeneration effect after damage has been declared on the creature with Regeneration. This includes when an attacking creature is about to be killed by a blocker, when a blocker is about to be killed by an attacker or when a creature is about to be killed by a damage-dealing or destruction spell. Creatures cannot be regenerated if their owner has sacrificed them, if an effect has returned them to their owner's hand, or if the effect destroying them specifies that its target cannot be regenerated.
  
  

Multiple Regenerations

• Unless specified on the card, a creature with Regeneration can be regenerated as many times as its owner can pay the mana cost. Creatures suffer no penalty from multiple regenerations -- they simply remain tapped and out of commission either until their owner's next turn begins or an untapping spell is cast on them.
  
  

Regeneration Effect

• Some cards can add the Regeneration ability to a creature. The simplest is the Green card simply called "Regeneration," an Enchantment which is attached to a creature permanently and enables its owner to regenerate the creature for a cost of one Green mana. Such cards are not "used up" when the creature is regenerated; they remain attached unless someone casts a spell removing or destroying them.