The Aitareya Upanishad is one of the oldest Upanishads. It belongs to the Aitareya Aranyaka of the Rigveda. The Upanishad was compiled from the 33 verses of the last three chapters (4,5 and 6) of the second Aranyaka.
Aitareya Upanishad, like other Upanishads of Hinduism, asserts the existence of Consciousness as Atman,the Self or Brahman. It contains one of the most famous expressions of the Vedanta, "Prajnanam Brahma" (Knowledge is Brahman/god/divine/holy), which is one of the Mahavakyas.
Aitareya Upanishad belongs to the Shakal branch of the Rig Veda. It has three Adhyayas (Chapters). In the first Adhyaya, there are three Khandas (sections) having 4, 5 and 14 Mantras respectively.
In the second Adhyaya, there is only one Khanda having 6 Mantras, while in the third Adhyaya also, there is only one Khanda having 4 Mantras. The total number of Mantras is 33.
Another special thing about Aitareya Upanishad is that it contains one of the four Maha Vakyas from the Vedas. The MahaVakya Prajnanam Brahma (प्रज्ञानं ब्रह्म) belongs to this rather short Upanishad. Prajna means awareness, consciousness. We do not need any proof to know the answer to the question “Do I exist?” We know it from within. Even in total darkness and with closed eyes, we know that “I am, I exist.” As we have this consciousness of I, we also have consciousness of Brahman. This consciousness, this knowledge is Brahman itself.