The Undead are dead who can still interact with the Living. By the Third Age, the term "Dwimmerlaik" was used as a catch-all term.
- Barrow-wights are not really undead, but evil spirits inhabiting the tombs of the Barrow-downs. They often possess the grave-gifts and bones of those entombed there, making them similar to the animate skeletons of other works.
- Ghosts (the Army of the Dead, specifically) are the souls of the dead who are prevented from reaching their final rest. The largest concentration were the dead warriors of Rohan who betrayed Isildur and were cursed, haunting the mountain until Aragorn persuaded them to fight for him. Their faces often shift from skeletal frames to pasty shadows of themselves.
- Nazgûl or Ring-Wraiths: Formerly Kings of Men, the nine rings gifted to them by Sauron caused them to become ghost-like Wraiths. After Glorfindel prophesied to Eärnur that the Witch-King would not fall to the hands of a man, it became true. The Witch King was only slain as he was weakened by a hobbit and then slain by a woman. If any male warrior scored a hit, they were affected by the Black Breath, a magical affliction characterised by stygian despair. They cannot see properly, and rely on smell and hearing. The Nazgûl carry daggers known as Morgul-blades meant to break off in the wound, which can turn the afflicted into a wraith. Frodo was stabbed by one at Weathertop. However, it is mentioned that Nazgul are not properly undead.
- Wraiths were humans, hobbits or elves (dwarves had natural resistance) who come into possession of a Ring of Power and wore it too long, or were stabbed by a Morgul-blade. They were permanently invisible, and more or less immortal.
- Phantasms were delusions and/or spirits or fallen Maiar. Morgoth and Sauron used them for their treachery. It is said Saruman also used them.
There may also have been some form of undead residing in the Dead Marshes
There are many of these fearsome creatures in the Barrow-downs where the Witch-king of Angmar himself went when he was looking for the One Ring in The Shire. For his evil presence, the ancient spirits were raised again killing people with their ceremonial swords and bringing the terror with them.