Amulet of life saving
If you are wearing an amulet of life saving and die, the amulet literally saves your life and restores full hit points. In the process the amulet disintegrates, and you lose 1 constitution point. If your hit point maximum had been reduced to a very small value, it is reset to either 10 or twice your experience level, whichever is higher. The beatitude of the amulet does not affect this effect in vanilla NetHack.
The amulet does not protect against death by self-genocide, death by brainlessness or forfeiture through trickery. While it will save your life if you teleport to a negative dungeon level, it will not save your game. Instead, you will simply escape the dungeon instead of dying.
There is a 7.5% chance that a randomly-generated amulet will be an amulet of life saving, making these amulets fairly rare. However, there is a guaranteed amulet of life saving on the middle level of Vlad's Tower.
Living monsters can wear amulets of life saving as well; they will come back to life after they die, using up the amulet but also identifying it for you, provided you saw the revivification happen. The amulet cannot protect monsters from genocide, although the amulet will still activate and be destroyed if a monster is wearing it when genocided. Non-living creatures (undead, golems, vortices and manes) cannot use the amulet. Monsters do not suffer brainlessness and so can be saved from death by monsters with intelligence drain attacks.
Enlightenment will give the message "Your life will be saved."
- But wait... Your medallion begins to glow! You feel much better! The medallion crumbles to dust!
- You died but were resurrected and the amulet is used up.
- But wait... Your medallion feels warm! You feel much better! The medallion crumbles to dust!
- As above, while blind.
- Unfortunately your brain is still gone.
- You died and were resurrected, and then die from brainlessness.
- Unfortunately you are still genocided...
- You died and were resurrected, and then die from genocide.
- An energized cloud of dust begins to coalesce. Your body rematerializes, and you gather up all your possessions.
- You were resurrected after teleporting to dungeon level 0. 
- But wait... <monster>'s medallion begins to glow! <monster> looks much better! The medallion crumbles to dust!
- A monster died wearing the amulet. The amulet is identified.
- But wait... <monster>'s medallion begins to glow! <monster> reconstitutes! The medallion crumbles to dust!
- A monster exploded wearing the amulet. The amulet is identified.
- Unfortunately <monster> is still genocided...
- A monster died and was resurrected, and then dies from genocide.
- Maybe not...
- You killed a life-saved monster out of direct sight. The amulet is not identified.
- But ... the chain on your medallion breaks and it falls to the <floor>! You hear homeric laughter!
- A cursed amulet of life saving did not save your life in UnNetHack.
- The chain on your medallion breaks, and you hear a faint giggling! Your medallion suddenly falls off! It doesn't look like you're going to make it after all...
- A cursed amulet of life saving did not save your life in SporkHack.
For an early character, an amulet of reflection may be a better choice, as reflection can save your life many times over (being life saved after a winter wolf cub's frost blast is no consolation if the winter wolf is still around).
The most common reason for an amulet not working is if you forgot to put it on.
Some players consider life saving an essential part of the ascension kit, but others call it the "amulet versus YASD" and point out that another amulet is much more useful with various armor combinations. It also increases the level of risk you can afford, which lowers resource consumption (particularly in the Endgame).
It can be extremely aggravating having your amulet of life saving stolen by a nymph; once she puts it on, killing her will save her life, disintegrating the amulet. If this happens to you, there are three ways you can retrieve your amulet:
- Kill (or tame) her before she puts it on.
- Once a monster wears an amulet, it will not give it up voluntarily. Before that however, the amulet is simply a normal item, and can be retrieved. Fortunately, nymphs do not currently wear things immediately after stealing. She will wear it the next time she picks any item up, or if she polymorphs, assuming the new form can put amulets on. If you kill her before either of these things happen, you can retrieve it. Alternatively, you can tame her; monsters drop any unused inventory items on taming, and this should include the amulet if unworn. Pets can use amulets however, so you'll need to get it back before she can.
- Polymorph into a nymph and steal it back.
- This has the added advantage that the nymph will not steal from you while you're polymorphed as a nymph.
- Polymorph the nymph known to be wearing an amulet of life saving until it becomes an undead creature, a golem, a vortex or a manes. (Perhaps tame her and use a nearby polymorph trap. The other means of polymorph risk killing her and using up the amulet.) Then, when you destroy the non-living monster, the amulet will be dropped.
Sometimes you will identify other monsters already wearing amulets of life saving, either by a means of treasure detection, a means of probing, or by seeing the monster pick the amulet up and put it on. Certain creatures are not living, and accordingly cannot be life saved. You can polymorph such amulet-wearing foes into a non-living creature, as described above for a nymph.
You will not gain any experience for "killing" a monster that is life saved, but will still break pacifist conduct.
NetHack keeps track of how many times your life has been saved by an amulet of life saving. If this number is zero, and you end your game without dying (i.e. by ascending, escaping or quitting), your end-of-game attributes list will include the line “You survived.” This is referred to as the survivor semi-conduct. If the number is not zero, the line will instead read “You survived after being killed [n times].”
Surviving is usually an indicator that the player was lucky or skillful enough not to die during a particular game. In particular, survivor can add prestige to a foodless ascension, because it removes one of the game's alternative sources of nutrition. Exploiting the polyself bug to raise maximum hit points is incompatible with surviving.
In UnNetHack and SporkHack, a cursed amulet of life saving is not guaranteed to save your life; it will do so with a 3/4 chance in UnNetHack and with a 1/2 chance in SporkHack. Additionally, if you are not lifesaved in SporkHack, your death will be logged as "killed by a cursed amulet of life saving" instead of your actual killer.