The Undying Lands were a realm inhabited by Ainur and Elves. The area included the continent of Aman and the island of Tol Eressëa. The ocean Belegaer separated the Undying Lands from the western shores of Middle-earth.

Originally, mortals were allowed to trade with those from Valinor in the Undying Lands, but were forbidden to sail west beyond the sight of Numenor. However, in SA 3319 (Second Age), the treacherous Sauron deceived Ar-Pharazôn, the last King of Númenor, to believe that the ruler of the Undying Lands would be granted immortality and persuaded him that this was man's right and he had but to claim it. To prevent the king's invasion, Ilúvatar destroyed Númenor beneath the ocean and set the Undying Lands forever beyond the reach of mortal Men. Elves were still permitted to sail across the sea to the Undying Lands, if they chose to (which most did).

In TA 3021 (Third Age), Círdan the Shipwright accompanied Elrond, Galadriel, and Gandalf, the Keepers of the Rings, on a voyage to the Undying Lands, where they intended to remain. They were also joined by Frodo and Bilbo Baggins, Hobbits who were among the very few mortal beings to be allowed passage to the Undying Lands.

Eventually, Samwise Gamgee, another Hobbit of the Shire, and the Dwarf Gimli along with his good friend Legolas, also made the journey. There have been suggestions that Aragorn, King of Gondor and Arwen later entered the Undying Lands, however this is not true.

It is tradition among the elves that Tuor and Idril made a journey there when Tuor was old, and were permitted to remain there.

In his novels, J.R.R. Tolkien made a point of establishing that mortals remain mortal, even if they are permitted to enter the Undying Lands (excepting Tuor, who, according to tradition, was allowed by Eru to join the Elves and became immortal as they are.)