The Upanishads (upanishad; Devanagari) are part of the Hindu Shruti scriptures which primarily discuss meditation and philosophy and are seen as religious instructions by most schools of Hinduism.

The Upanishads are mystic or spiritual interpretations on the Vedas, their putative end and essence, and thus known as Vedānta ("the end of the Vedas"). The Sanskrit term upanishad derives from upa- (near), ni- (down) and shad (to sit), i.e. referring to the "sitting down near" a spiritual teacher (guru) in order to receive instruction in the Guru-shishya tradition or parampara. The teachers and students appear in a variety of settings (husband answering questions about immortality, a teenage boy being taught by Yama, etc.). Sometimes the sages are women and at times the instructions (or rather inspiration) are sought by kings.

Different Upanishad serve as commentaries or extensions of each of the four Vedas (Rigveda, Yajurveda, Samaveda and Atharvaveda). The longest Upanishad are the Brihadaranyaka and the Chāndogya.

According to tradition they were transmitted orally and at the end of Dvapara Yuga written down by Vyasa. Their language is generally Classical Sanskrit, although the very oldest of them may be considered to be written in Vedic Sanskrit. Taken together, they do not date to a particular epoch, but the most recent ones were written in Early Modern or Modern times (depending on the definition of the canon, some schools accept up to 350 Upanishads). The oldest Upanishads are the Brihadaranyaka and the Chhāndogya ones, dating to the "Brahmana" period of Vedic Sanskrit (from roughly 900 BCE). A second stratum belongs to the "Sutra" period (from roughly 600 BCE), including the Katha and Maitrayai ones.

Reputedly, there were over two hundred Upanishads, but the philosopher Shankara only considered fifteen or so to be primary. The Muktika Upanishad lists 108 Upanishads, accepted as shruti by the Advaita school, but only 12 of them are accepted by all Hindus. The Upanishads were not fully recorded until 1656, at the order of Dara Shikoh.


The following is a list of the ten "principal" (mukhya) Upanishads that were commented upon by Shankara, and that are accepted as shruti by all Hindus

"Principal" Upanishads
The following is a list of the ten "principal" (mukhya) Upanishads that were commented upon by Shankara, and that are accepted as shruti by all Hindus. They are listed with their associated Veda (Rigveda), Samaveda , White Yajurveda , Black Yajurveda , Atharvaveda ).

Aitareya
Brihadaranyaka
Ishopanishad
Taittirīya
Katha Upanishad
Chāndogya
Kena
Mundaka Upanishad
Mandukya Upanishad
Praśna


The Kauśītāki, Śvetāśvatara and Maitreya Upanishad are sometimes added to extend the canon to 12 or 13. They are also the oldest Upanishads, likely all of them dating to before the Common Era.