The present volume contains the Text, Translation, important . Notes and exhaustive Commentary on Nine Principal Upanishads. Originally, these Principal Texts appeared in two volumes in earlier editions, and sometime after the publication of the same, these works of His Holiness Sri Swami Sivanandaji Maharaj went out of print for many years. For some reason or other, the reprinting of these famous Scriptures with the Commentary of His Holiness could not become possible, though students of the Prasthanatraya,-the triple foundation of Indian Philosophy, viz., the Upanishads, the Brahmasutras and the Bhagavadgita,- were eager to . see the coming out of these treasures of writing and were actually communicating to us their requests in this regard incessantly.