The Upanishads are the concluding portion of the Vedas — hence their other name, Vedanta, “the end of the Vedas.” Where the earlier layers of the Vedas concern ritual, hymn, and cosmic order, the Upanishads turn inward, asking a single relentless question in dozens of different ways: what is the true nature of the self, and how does it relate to ultimate reality?
Composed over centuries by many different sages, and often recorded as dialogues between teacher and student, the Upanishads are less a single treatise than a living conversation. The Chandogya Upanishad’s teaching “Tat Tvam Asi” — “That Thou Art” — collapses the imagined distance between the individual self and Brahman. The Katha Upanishad frames a boy’s dialogue with Yama, lord of death, as a meditation on what actually survives the body. The Brihadaranyaka Upanishad records Yajnavalkya’s debates at the court of King Janaka, considered among the most sophisticated philosophical arguments in the ancient world.
Traditionally there are 108 Upanishads, though ten to thirteen are usually treated as the principal ones studied by serious students of Vedanta. Their central claim — that the innermost self, Atman, is not different from the ultimate ground of reality, Brahman — became the philosophical spine of most later schools within Sanatan Dharma, and continues to be studied, debated, and translated into new languages today.