The Illinizas are a pair of mountains that are located to the south of Quito. They are found in the ecological reserve that shares its name with the mountains. These twin mountains are separated in the middle by a saddle that is about a kilometer long. The peaks are among the highest in Ecuador, with Illiniza Sur standing slightly taller than Illiniza Norte, its northern counterpart, at 5248 metres and 5126 metres respectively.

Most guidebooks (for example, Lonely Planet Ecuador, Ecuador: A Climbing Guide) spell the mountain with only one "l" as in Iliniza. The name Illinizas is derived from the Kunza words for "masculine hill."

Whilst Illiniza Sur (the southern peak) is a more difficult climb due to its glacial nature, Illiniza Norte requires little or no climbing expertise, and may be climbed as a trekking peak. A guide is still recommended, however, as the path becomes hard to navigate as it approaches the summit.

The Illinizas are generally thought to be the best acclimatization mountains in Ecuador, and are frequently used as a preparatory climb to higher peaks such as Cotopaxi, Chimborazo and Cayambe.^{[citation needed]}