Ithilien was a fair and prosperous land during the first part of the Third Age, when Gondor was strong and Mordor deserted, but later the people fled across the Anduin to escape war. The Stewards of Gondor still kept scouts in Ithilien, based on secret locations such as Henneth Annun.
In The Lord of the Rings, Gollum leads Frodo Baggins and Samwise Gamgee through Ithilien on the way to the pass of Cirith Ungol into Mordor. After witnessing a battle with men of Harad accompanied by oliphaunts, the hobbits are captured by Faramir, the son of the Steward Denethor, but are allowed to continue when he is satisfied they are not agents of Sauron.
During the Fourth Age, Ithilien was ruled by the Princes of Ithilien, a line that started with Faramir.