When it works
Teleport control works, regardless of whether it is an intralevel teleport or levelport, and regardless what caused you to teleport (teleportitis, wand, scroll, trap, quantum mechanic attack, or throne effect).
Teleport control also allows you to attempt to teleport to escape drowning if you have teleportitis or are a teleporting monster.
When it doesn't work
If you are stunned or unconscious, you will be unable to control your intra-level teleports.  If you are stunned you will be unable to control your levelports. If you are confused your levelports may take you to a random level instead of the one you pick ("Oops..."); this chance is dependent on your luck (rnl(5)).
If you select a region of the screen you cannot occupy (inside stone), or a square with a trap or boulder or already occupied by a monster, you will instead be sent to a random point in the level with the message "Sorry...".
Teleport control can be acquired extrinsically by:
- reaching experience level 17 as a wizard or monk
- polymorphing into a tengu
- eating a tengu(17%) or the Wizard of Yendor(25%).
- eating a ring of teleport control
Controlled level teleport
If you try to levelport to too deep a level--if you say "99", e.g.--you will be sent to the lowest possible level from your current position, which generally means the bottom of your current dungeon branch. A controlled levelport from anywhere in the Dungeons of Doom cannot take you deeper than the Valley of the Dead; a controlled levelport from anywhere in Gehennom can take you only as deep as the vibrating square level until you perform the invocation ritual.
If you try to levelport to too shallow a level, you will be sent to the parent dungeon branch. For example, the same level one is reachable from the Dungeons of Doom, Gehennom, the Gnomish Mines, and Vlad's Tower. In the Quest, levels are counted relative to the quest branch, in other words you should use the number displayed on the bottom line (like "Home 1"), instead of the depth displayed on restoring ("You return to level 16 in the Quest"). You cannot levelport out of the quest, because negative numbers are still interpreted as absolute levels (0=suicide, 1-8 falling down and dying, 9++ heaven). In Fort Ludios you cannot levelport at all (exept to negative numbers, with the same bad effects as everywhere  ).
- "You feel controlled!"
- You gained teleport control through level gain
- "You feel uncontrolled!"
- You lost teleport control through level drain
- "You feel in control of yourself."
- You gained teleport control through eating a corpse
- "You feel centered in your personal space."
- You gained teleport control through eating a corpse, while hallucinating
In SLASH'EM, it can also be acquired intrinsically by:
- eating the corpse of a giant shoggoth(20%).
In dNethack, it can be obtained by eating the corpse of a phase spider. (unknown chance)