The philosopher and commentator Shankara is thought to have composed commentaries on eleven mukhya or principal Upanishads, those that are generally regarded as the oldest, spanning the late Vedic and Mauryan periods. The Muktika Upanishad (predates 1656) contains a list of 108 canonical Upanishads and lists itself as the final one. Dara Shikoh (d. 1659), son of the Mughal emperor Shah Jahan, translated fifty Upanishads into Persian. Max Müller (1879) was aware of 170. Sadhale, in his massive verse index , has drawn on 223 different extant texts that call themselves by this name. Additionally, parts of earlier texts, of Brahmanas or passages of the Vedas themselves, are sometimes considered Upanishads.
The Upanishads speak of a universal spirit (Brahman) and an individual soul, (Atman) and at times assert the identity of both. Brahman is the ultimate, both transcendent and immanent, the absolute infinite existence, the sum total of all that ever is, was, or shall be. The mystical nature and intense philosophical bent of the Upanishads has led to their explication in numerous manners, giving birth to three main schools of Vedanta. Shankara's exegesis of the Upanishads does not describe Brahman as the God in a monotheistic sense; he ascribes to it no limiting characteristics, not even those of being and non-being. Thus, Shankara's philosophy is named advaita, "not two" as opposed to dvaita, founded by Madhvacharya, which holds that Brahman is ultimately a personal God, to be aligned with Vishnu, or Krishna (brahmano hi pratisthaham, I am the Foundation of Brahman Bhagavad Gita 14.27). The third major school of Vedanta is Vishishtadvaita, founded by Ramanujacharya and it has some aspects in common with the other two.
The ninth chapter of the Taittiriya Upanishad says:
The key phrase of the Upanishads, to Advaita Vedanta, is तत् त्वं अिस "Tat Tvam Asi" (That thou art). Vedantins believe that in the end, the ultimate, formless, inconceivable Brahman is the same as our soul, Atman. We only have to realize it through discrimination. (However, interpretations of this phrase differ.) Verses 6, 7 & 8 of Isha Upanishad:
The Upanishads also contain the first and most definitive explications of the divine syllable Aum or OM, the cosmic vibration that underlies all existence. The mantra "Aum Shanti Shanti Shanti" (the soundless sound, peace, peace, peace)is often found in the Upanishads. Devotion to God is foreshadowed in Upanishadic literature, and was later realized by texts such as the Bhagavad Gita.
The and Upanishads are sometimes added. All these date from before the Common Era. From linguistic evidence, the oldest among them are the and Chāndogya Upanishads. The Jaiminīya Upaniṣadbrāhmaṇa, belonging to the late Vedic Sanskrit period, may also be included. Of nearly the same age are the Aitareya, Kauṣītaki and Taittirīya Upaniṣads, while the remnant date from the time of transition from Vedic to Classical Sanskrit.
The older Upanishads are associated with Vedic Charanas, Shakhas or schools; the Aitareya and Upanishads with the Shakala shakha, the Upanishad with the Kauthuma shakha, the Kena Upanishad with the Jaiminiya shakha, the Upanishad with the Caraka-Katha shakha, the and Upanishads with the Taittiriya shakha, the Upanishad with the Maitrayani shakha, the and Upanishads with the Vajasaneyi Madhyandina shakha, and the and Upanishads with the Shaunaka shakha.
In the Muktika Upanishad's list of 108 Upanishads the first 10 are grouped as mukhya "principal". 21 are grouped as Sāmānya Vedānta "common Vedanta", 23 as Sannyāsa, 9 as Shākta, 13 as Vaishnava, 14 as Shaiva and 17 as Yoga Upanishads.
Later Upanisads are often highly sectarian: this was "one of the strategies used by sectarian movements to legitimate their own texts through granting them the nominal status of Śruti. For the most part, the canonical Shakta Upanishads are sectarian tracts reflecting doctrinal and interpretative differences between the two principal sects of Srividya upasana (a major Tantric form of Shaktism). As a result, the many extant listings of "authentic" Shakta Upanisads vary in content, reflecting the sectarian bias of their compilers:
"Past efforts to construct lists of Shakta Upanisads have left us no closer to understanding either their 'location' in Tantric tradition or their place within the Vedic corpus. [...] At stake for the Tantric is not the authority of sruti per se, which remains largely undisputed, but rather its correct interpretation. For non-Tantrics, [it is a text's] Tantric contents that brings into question its identity as an Upanisad. At issue is the text's classification as sruti and thus its inherent authority as Veda."
Of the texts listed in the Muktika Upanishad nine are classified as Shakta Upanishads:
The list excludes several notable and widely used Shakta Upanisads, including the , the and the .