The greatness and the sublimity of the Upanishads are well known to all the students of philosophy. There have been attempts to approach the books through various standpoints. Much has been written over the knotty problems of interpretation, by the Eastern and Western scholars. And yet the lay reader has not understood the central teachings fully well. Gurudev Sri Swami Sivanandaji Maharaj, in his comprehensive volume The Principal Upanishads has given exhaustive commentary on Nine Upanishads and stressed such points clearly and truly, explaining the abstruse ideas in his own inimitable style, thus laying bare the sacred doctrine not only before the eligible pupil but also the lay reader. For the convenience of the readers, we are bringing out each Upanishad in a separate book. The present volume contains the text, translation, notes, and commentary on Aitareya Upanishad.