The regeneration ability always has a cost and an effect. For example in Troll Ascetic it says: 1G: Regenerate Troll Ascetic That means that if you pay 1G you will put a 'regeneration shield' over your Troll Ascetic. Any combat damage or burn damage (such as schock, or volcanic hammer) will deplete the 'regeneration shield' instead of killing your Troll (assuming that somehow they can target the Troll). As an effect of using the 'regeneration shield', the regenerated creature becomes tapped. Now damage done by attacking and blocking creatures. Once you declared blockers, the next step in the combat phase is called "combat damage". Here you assign the damage from the creatures you control and your opponent does the same. Once damage is assigned, you (and your opponent) are allowed to play effects, abilities or spells, that will go on a stack. Once the stack solves and its empty, damage will be done. Lets say you attack with Troll ascetic and I block with a grizzly bears. You assign 3 damage to my bears, and I assign 2 damage to your Troll. Damage goes on the stack. I play giant growth on my bears. You play your regeneration ability (damage has been assigned but not yet done). Once we both pass and damage is dealt. Your Troll will be tapped, since you used its regeneration shield. One more thing about regeneration: you can pay the regeneration shield as many times as you want, and not necesarrilly use it. Lets say that you have magus of the vineyard and no way to spend that mana, you can use it on your Troll Ascetic's regeneration, even if it doesnt get damage at all.