There are more than 200 known Upanishads, one of which, Muktikā Upanishad, predates 1656 CE and contains a list of 108 canonical Upanishads, including itself as the last. The earliest ones such as the Brihadaranyaka and Chandogya Upanishads date to the 1st millennium BCE, and the latest to the Mughal period. Various schools of Hinduism recognize the first 10, 11, 12 or 13 Upanishads as "principal" or Mukhya Upanishads. The remainder is further divided into Upanishads associated with Shaktism, Sannyasa (asceticism), Shaivism, Vaishnavism, Yoga, besides 21 Upanishads known as sāmānya ("common", or "general") which, while not part of the mukhya canon are still accepted as shruti by all schools of Vedanta. The newer Upanishads mentioned in the Muktikā probably originated in southern India. They are also categorized as "sectarian" since they reflect the emergence of the various Hindu sects in medieval Hinduism which sought to legitimize their texts by claiming for them the status of Śruti. The Upanishads of the Muktika canon are also all associated with a specific Brahmana and by extension with one of the four Veda.
The Mukhya Upanishads can be grouped into periods. Of the early periods are the Brihadaranyaka and the Chandogya, the oldest.[note 6]
The Aitareya, Kauṣītaki and Taittirīya Upanishads may date to as early as the mid 1st millennium BCE, while the remnant date from between roughly the 4th to 1st centuries BCE, roughly contemporary with the earliest portions of the Sanskrit epics. It is alleged that the Aitareya, Taittiriya, Kausitaki, Mundaka, Prasna, and Katha Upanishads show Buddha's influence, and must have been composed after the 5th century BCE, but it could just as easily have been the other way around. It is also alleged that in the first two centuries A.D., they were followed by the Kena, Mandukya and Isa Upanishads, but other scholars date these earlier. Not much is known about the authors except for those, like Yajnavalkayva and Uddalaka, mentioned in the texts. A few women discussants, such as Gargi and Maitreyi, the wife of Yajnavalkayva, also feature occasionally.
Each of the principal Upanishads can be associated with one of the schools of exegesis of the four Vedas (shakhas). Many Shakhas are said to have existed, of which only a few remain. The new Upanishads often have little relation to the Vedic corpus and have not been cited or commented upon by any great Vedanta philosopher: their language differs from that of the classic Upanishads, being less subtle and more formalized. As a result, they are not difficult to comprehend for the modern reader.
|Rig Veda||Only one recension||Shakala||Aitareya|
|Sama Veda||Only one recension||Kauthuma||Chāndogya|
|Yajur Veda||Krishna Yajur Veda||Katha||Kaṭha|
|Taittiriya||Taittirīya and Śvetāśvatara|
|Shukla Yajur Veda||Vajasaneyi Madhyandina||Isha and Bṛhadāraṇyaka|
|Atharva||Two recension||Shaunaka||Māṇḍūkya and Muṇḍaka|
The Kauśītāki and Maitrāyaṇi Upanishads are sometimes added to the list of the mukhya Upanishads.
There is no fixed list of the Upanishads as newer ones have continued to be discovered and composed. On many occasions, when older Upanishads have not suited the founders of new sects, they have composed new ones of their own. 1908 marked the discovery of four new Upanishads, named Bashkala, Chhagaleya, Arsheya and Saunaka, by Friedrich Schrader, who attributed them to the first prose period of the Upanishads. The text of three, the Chhagaleya, Arsheya and Saunaka, was reportedly corrupt and neglected but possibly re-constructible with the help of their Perso-Latin translations. Other texts including Devadeva-rahasya and Subakshana have also ascribed as Upanishads. Several texts under the title of "Upanishads" originated right up to the first half of the 20th century, some of which are not to be included in the Vedas, since they did not deal with subjects of Vedic philosophy.
The main Shakta Upanishads mostly discuss doctrinal and interpretative differences between the two principal sects of a major Tantric form of Shaktism called Shri Vidya upasana. The many extant lists of authentic Shakta Upaniṣads vary, reflecting the sect of their compilers, so that they yield no evidence of their "location" in Tantric tradition, impeding correct interpretation. The Tantra content of these texts also weaken its identity as an Upaniṣad for non-Tantrikas and therefore, its status as shruti and thus its authority.