The wizard is one of the roles in NetHack. Wizards can be either neutral or chaotic, and can be humans, elves, gnomes or orcs. They are relatively poor fighters, but they are the best spellcasters in the game. The guidebook says of them:
Wizards start out with a knowledge of magic, a selection of magical items, and a particular affinity for dweomercraft. Although seemingly weak and easy to overcome at first sight, an experienced Wizard is a deadly foe.
- 1 Starting equipment
- 2 Intrinsics
- 3 Skills
- 4 Special rules
- 5 Strategy
- 6 Rank titles
- 7 Quest
- 8 Variants
- 9 Encyclopedia entry
- 10 References
A wizard begins the game with:
- a blessed +1 quarterstaff
- a cloak of magic resistance
- three random scrolls
- a blessed spellbook of force bolt
- a random non-force bolt spellbook of level 3 or lower 
- three random potions
- two random rings
- one random wand
- if an Elf, random non-magical musical instrument
- 20% chance of magic marker
- 20% chance of blindfold
The random items generated will never be any of the following: 
- scroll of fire
- scroll of amnesia
- scroll of blank paper
- spellbook of blank paper
- potion of hallucination
- potion of acid
- ring of aggravate monster
- ring of hunger
- ring of levitation
- any ring with an enchantment of +0 or lower 
- ring of polymorph control if you already have a wand, ring, or potion of polymorph
- the ring or potion of polymorph if you already have a ring of polymorph control
- wand of wishing
- wand of nothing
- Exp level 8: teleport at will if the character has teleportitis
- Exp level 15: warning
- Exp level 17: teleport control
Wizards start always with Basic skill in Quarterstaff, Attack spells and Enchantment spells. This is independent of the category of the random spellbook, because the spellcasting skills are hardcoded in the function skill_init in WEAPON.C.
Skill slot management
It takes 6 skill slots to reach Expert in daggers, and 20 slots to max out all the spellcasting skills: 3x2 slots for the Skilled spells, 4x4 slots for the Expert spells, minus the 2 points already in attack and enchantment. This means that a level 30 Wizard who chooses to enhance all spellcasting skills to maximum should have 3 slots free to spend on other skills, such as unrestricted artifact weapon skills.
As part of skill slot management, a Wizard should defer enhancing any spell skill until there comes a point when doing so would provide an in-game benefit. The benefits from enhancing a spell skill are that it lowers spell failure rates, and (depending on the spell) sometimes augments the effects of a spell. When advancing spell skills, it is a good idea to check which spells you have in each school, and make sure that at least one of your spells would benefit from being enhanced, before using up the skill slot.
It is important to note that a great many spells enjoy augmented effects when advancing from Basic to Skilled (notably, fireball, cone of cold, remove curse, and detect monsters), but only two spells enjoy augmented effects when going from Skilled to Expert (namely, jumping and protection), and of these two, Wizards can only become Expert in jumping. Therefore, for spells other than escape spells, a Wizard gains no benefit from advancing to Expert unless there is a currently known high level spell (e.g. finger of death, cancellation, polymorph) whose failure rate would be lowered by such advancement.
In particular, there is almost never any need to advance divination spells to expert, since all divination spells have the same effects at skilled as at expert, and none of the divination spells are of very high spell level. The same goes for enchantment spells and healing spells (except for the useless restore ability). The skill slots saved thereby are valuable to have available for weapons, especially early in the game.
Wizards can write scrolls and spellbooks they don't know yet with much higher success rates than other roles. With maximized Luck, writing one is nearly a certainty.
Wizards with an Intelligence of 15 or 16 have reduced-hunger casting, and wizards with an Intelligence of 17 or above have hungerless casting.
Wizards get a warning when they have a high chance of failing to read a spellbook: This spellbook is [very] difficult to comprehend. Continue?
Some people advocate deliberately keeping your experience level low in the early game, but this is controversial. Increases in monster difficulty due to a higher experience levels are roughly balanced out with the benefits that come with the higher experience level, such as higher maximum power. The easiest way to prevent yourself from over-leveling is to allow your pet to kill as many monsters as is feasible, although one would be wise to still gain a few levels for the additional survivability and spell power capacity. These constraints are loosened when you learn magic missile, get Magicbane, or obtain some other offensive upgrade.
Unless you are playing an atheist, if you manage to find an early coaligned altar, consider sacrificing at it until you get Magicbane, which will make your early game a lot easier.
The Mines are dangerous. Unless you are a gnome, all the normal inhabitants will want to kill you. It can be quite hard for inexperienced players to go straight to Minetown and survive long enough to return to the main dungeon. Exploring the deeper Mines past Minetown should perhaps be postponed, though you do eventually want to get there, as the Catacombs can be very profitable for a Wizard.
Completing Sokoban before doing anything else is likewise dangerous, because monsters difficulty increases with each completed puzzle, and your own experience level will probably not keep up with it.
Consider going down to the Oracle level and breaking the centaur statues there in search of spare spellbooks, gaining a few levels in the process, then coming back up to do the Gnomish Mines.
Hunger may become an issue, especially if you do not start the game with Intelligence of 17 or above for hungerless casting. You may need to pray for food when Weak, and the first floor of Sokoban with its guaranteed food might be a good place to go to stock up.
Early game Wizards are very weak and should avoid engaging in melee combat with monsters. Finding a source of thrown weapons to damage or kill monsters before they can reach you is critical.
Most Wizards pursue daggers as their sole weapon skill, because it is easy for them to get Magicbane (which uses dagger skill), and because throwing daggers at Expert skill later in the game does considerably more damage than using a quarterstaff. To that end, training daggers as soon as you come across some is a good idea. Easy ways to train daggers include throwing them at slow enemies, letting a fog cloud engulf you, or naming Sting and training it on goblins for the +d5 to-hit bonus. Descending briefly into the Gnomish Mines, where daggers are plentiful, is a good way to acquire some.
Still, in the very early game, daggers may be hard to come by, so consider casting force bolt at a boulder or statue, picking up the resulting stones (as many as can be carried while staying unburdened), and quivering them. Throwing rocks at a monster is better than nothing, especially if you're out of Power. Of course, you should replace them with darts, daggers, or other missiles as they become available.
Some players advocate dumping the starting quarterstaff early on, either as soon as you find a non-cursed dagger or you reach Basic skill in daggers. There is little reason to keep the staff after this point unless you plan to try and wish for the Staff of Aesculapius, in which case you should continue training quarterstaff skill.
Players disagree on whether Wizards should wear high AC metal armor and focus on melee skills, or ignore all metal armor in favor of being able to use spells in combat. Wearing metal is fine if it doesn't encumber you and you're confident in your ability to survive without relying on magic. Not wearing it is fine if you're prepared to flee from monsters when you run low on energy. If you do choose to wear metal, you should plan to swap it out later for nonmetal armor as that gradually becomes available.
Save your energy for when you really need it. You won't be able to cast force bolt more than a few times until you gain a few levels.
Read any non-cursed spellbook as soon as you find it. You will always get a warning if there's a chance of failing to read it, and if you do, it's likely to be too high level to be of much use right now.
If you start with a wand of polymorph, you may consider polymorphing your starting spellbooks. Your force bolt book, since it is blessed, will polymorph into another blessed book that can thus be read with a 100% success rate. However, this can be done only once or twice before the spellbook becomes too faint to read. You can also polymorph your starting kitten at the same time, often ending up with something much stronger for an early pet (though this risks killing it and angering your god).
Raising your Luck and maintaining it should be a priority. Finish the Mines and Sokoban when you feel ready.
The Wizard quest is fairly easy, the Dark One is a pushover, and the reward - the Eye of the Aethiopica - is so fantastically useful that a Wizard should probably go on the quest as soon as possible. One of the many things the Eye does is vastly enhance strategic mobility through the Mazes of Menace - in particular, you can get out of trouble in an instant. Many Wizards set up a base camp in Sokoban and leave most of their worldly possessions there, going back whenever they need to read a spellbook or pick up more food. Note that adjacent monsters may follow you through the Eye's magical portal, but this also has its uses, such as luring Wraiths out of a graveyard level.
If you have found the quest portal, but are not yet level 14, be aware that the first level of the quest, the Lonely Tower, is often well-provided with wraiths. You may be able to gain a few levels and reach XL 14 before you meet your quest leader.
For Wizards with the Eye and the magic missile and create monster spells, altar farming is incredibly easy, since an arbitrary number of corpses for sacrifice can be created on demand. This is good if you want to get Magicbane or other favors from your god. If you decide to get crowned, you will receive a spellbook of finger of death.
You can and should train daggers to Expert skill. At this skill level, it is possible to throw up to three daggers in a turn, so it is recommended to acquire a good number of daggers that stack together. Elven daggers are especially useful. Note however, in 3.6.x there is no +1 multishot at skilled for a wizard so you will never throw more than two daggers or darts per turn at expert.
Acquire Magicbane at this stage, unless you have some reason not to.
By now you should be accumulating a solid library of spells. If you have come across a magic marker, you can blank some of the more useless spellbooks and write useful ones like magic missile and identify.
Since so many monsters are resistant to one or more forms of magical attack, you will need to build a diverse toolkit. A fire resistant opponent may be especially vulnerable to cold attacks; an opponent with reflection can probably be brought down by splash effects from a fireball. Reading the wiki entries for new opponents to learn their immunities and weaknesses is a good idea. Ultimately, nothing has "stab resistance" - even shades will succumb to an enchanted or silver weapon - so a wizard should always have a good physical combat option to fall back on. Still, spells are generally the most powerful means of attack by this point in the game.
Some players prefer to raise their experience level as high as possible in order to have the largest maximum HP and energy, and to maximize damage from casting magic missile. A level 30 wizard gets 16d6 damage with each magic missile. Luring wraiths out of the Valley of the Dead and other graveyards will probably be required to reach this level. Remember to buy all the divine protection you want before trying to level up.
Other players like to keep their experience level low, to keep higher-difficulty monsters from being generated. Ultimately, the choice is up to you.
Magicbane remains an excellent weapon. With its magic resistance and ability to catch curses, you can fight powerful spellcasters like arch-liches head-on. Opinions vary as to whether you should enchant it to +7 or keep it at +2; see Magicbane#Enchanting for details.
In any case, you should generally rely on spells as your main attacks by now, and only engage monsters in melee when you need to conserve energy.
The helm of brilliance is a favourite helmet for spellcasters, and wizards should get one. Spellbooks will be much easier to read, and failure rates for high-level spells will drop enough to make them feasible. Additionally, if wearing a helm of brilliance brings the sum of your Intelligence and Wisdom to 45 or above, the amount of energy you recover will be boosted from 1d3 to 1d4. This doesn't seem like much at first, but since you recover energy every turn while carrying the Eye of the Aethiopica, it is quite a substantial amount.
Casting spells with the Amulet can drain your power very fast in the End Game. You can drop it on the ground when you need to cast a lot of spells without moving (this is a bad idea in UnNethack, which teleports the AoY to a random location on the level anytime it is dropped). Also, consider stocking up on a few charging scrolls and reading them when confused to restore all your energy immediately.
The status line shows you to be one of the following ranks when you reach the specified experience level:
- XL 1-2: Evoker
- XL 3-5: Conjurer
- XL 6-9: Thaumaturge
- XL 10-13: Magician
- XL 14-17: Enchanter/Enchantress
- XL 18-21: Sorcerer/Sorceress
- XL 22-25: Necromancer
- XL 26-29: Wizard
- XL 30: Mage
SLASH'EM adds the Rank titles Warlock/Witch (XL 14-17), bumps Enchanter/Enchantress to XL 18-21, Sorcerer/Sorceress to XL 22-25, and removes the rank title of Necromancer (presumably, to avoid confusion with the new Necromancer role).
In SLASH'EM, Wizards start with four spellbooks, with one book chosen randomly from each of:
- force bolt or sleep
- resist poison or resist sleep
- detect food, detect monsters, light, knock or wizard lock
- magic missile, confuse monster, slow monster, cure blindness, endure heat, endure cold, insulate, create monster or healing
Wizards in SLASH'EM are a bit more difficult then in vanilla. In vanilla, once you get Magicbane you are likely to be on the road to ascension. Here, you had better stay sharp even after you get Magicbane. Keep your pet around you for longer. A good advice would be to stay careful and keep your pet until AC <-10, level >10.
An additional difficulty is that you don't have enough slots for all the extrinsics you would want. Dragon scale mail interferes with spell casting now and aren't worth it for the Ascension kit. In addition you have to deal with the create pool spell and more serious level drain attacks.
Some possible equipment choices:
- water walking boots: You need these to deal with create pool spells. You can use haste self as a substitute to speed boots. The amulet slot is alredy crowded for other things, so no amulet of flying for you.
- ring of levitation with speed boots: With the ring, you are vulnerable to the electric bugs. So you might want to blessed genocide "x" to be safe, and this is no joke. The spell of levitation is dangerously unreliable (unless skilled or better in escape spells).
- robe of protection: The highest natural AC for the suit slot without spellcasting interference. It can be upgraded from plain robes with 50% chance. The robe of power is useless for a Wizard, unless wearing metallic armor. Robes take the suit slot here, so you can't attempt to wear a robe of power on top of dragon scale mail.
- amulet of reflection or amulet of drain resistance : You'll need to give up on the amulet of flying and life saving.
- Wielding the hand of Vecna: It confers level drain resistance. You can rely on remove curse spell for the curses. Be careful of the blast if you are neutral.
- Wielding Stormbringer if chaotic for level drain resistance. You can obtain easily Stormbringer by crowning while carrying the spellbook of finger of death, this will also unrestrict broadsword skill. Without already carrying finger of death spellbook, you'll get that instead.
- Wielding Nighthorn: It confers reflection, but it is two-handed, so if it get cursed, you can't cast spells any more. Also, it is lawful, it will blast you for sure.
- Depending on your situation you could live dangerously, and forfeit reflection or drain resistance.
- Wielding the Staff of Aesculapius: It doesn't confer drain resistance in SLASH'EM. Furthermore, it is two-handed, if it get cursed, you can't cast spells any more.
- Wearing a shield of reflection: Your success rate with spells will not be higher than 55% for magic missile spell and 25% for anything else. Ever.
Ebenezum walked before me along the closest thing we could
find to a path in these overgrown woods. Every few paces he
would pause, so that I, burdened with a pack stuffed with
arcane and heavy paraphernalia, could catch up with his
wizardly strides. He, as usual, carried nothing, preferring,
as he often said, to keep his hands free for quick conjuring
and his mind free for the thoughts of a mage.