Upanishad merupakan salah satu daripada tiga sumber utama mazhab-mazhab Vedanta, di samping Bhagavadgita dan Brahmasutra. Pelbagai intepretasi berlainan dapat diberikan kepada ajaran-ajaran Upanishad. Amnya, mazhab-mazhab Vedanta cuba untuk menjawab persoalan mengenai perhubungan antara Brahman dan Atman, dan perhubungan antara Brahman dan dunia. Oleh itu, mazhab-mazhab Vedanta dinamakan mengikut pandangannya terhadap perhubungan antara Brahman dan Atman.

• Menurut Advaita Vedanta, tiada sebarang perbezaan antara Brahman dan Atman.
• Menurut Vishishtadvaita, Atman individu adalah sebahagian daripada Brahman, dan maka, ianya serupa tetapi bukan satu.
• Menurut Dvaita Vedanta, semua Atman individu (Jivatman) adalah entiti-entiti yang berasingan dan tidak sama dengan Brahman.

Lain-lain mazhab Vedanta termasuklah Dvaitadvaita oleh Nimbarka, Suddhadvaita oleh Vallabha, dan Acintya Bhedabheda oleh Sri Krishna Chaitanya.

Sankaracharya telah memberikan ulasan untuk 11 Upanishad utama.

Advaita Vedanta

Advaita secara literal bermaksud "ketidakduaan". Ini adalah suatu sistem pemikiran monistik. Brahman dan Atman (Diri) bukan dua entiti berlainan.

Antara semua mazhab Vedanta, Advaita Vedanta merupakan mazhab falsafah India yang paling berpengaruh. Pelopor falsafah ini ialah Gaudapada yang memberikan prinsip-prinsip asas falsafah Advaita dalam ulasannya terhadap ayat-ayat bercanggah Upanishad. Idea-idea ini, kemudiannya diperkembangkan oleh Sankaracharya.

Menurut King, karya utama Gaudapada, Māṇḍukya Kārikā, penuh dengan terminologi falsafah dan analogi agama Buddha. Menurut King juga, penulisan Sankaracharya mempunyai perbezaan ketara dengan Brahmasutra, dan bahawa idea-idea utama Sankaracharya bercanggah dengan idea-idea Upanishad. Sebaliknya, S.Radhakrishnan berpendapat bahawa, pandangan Sankaracharya adalah teru7s

It deals with the non-dual nature of Brahman and Atman. Advaita is considered the most influential sub-school of the Vedanta school of Hindu philosophy. Gaudapada was the first person to expound the basic principles of the Advaita philosophy in a commentary on the conflicting statements of the Upanishads. Gaudapada's Advaita ideas were further developed by Shankara (8th century CE). King states that Gaudapada's main work, Māṇḍukya Kārikā, is infused with philosophical terminology of Buddhism, and uses Buddhist arguments and analogies. King also suggests that there are clear differences between Shankara's writings and the Brahmasutra, and many ideas of Shankara are at odds with those in the Upanishads. Radhakrishnan, on the other hand, suggests that Shankara's views of Advaita were straightforward developments of the Upanishads and the Brahmasutra, and many ideas of Shankara derive from the Upanishads.

Shankara in his discussions of the Advaita Vedanta philosophy referred to the early Upanishads to explain the key difference between Hinduism and Buddhism, stating that Hinduism asserts that Atman (soul, self) exists, whereas Buddhism asserts that there is no soul, no self.

The Upanishads contain four sentences, the Mahāvākyas (Great Sayings), which were used by Shankara to establish the identity of Atman and Brahman as scriptural truth:

• "Prajñānam brahma" - "Consciousness is Brahman" (Aitareya Upanishad)
• "Aham brahmāsmi" - "I am Brahman" (Brihadaranyaka Upanishad)
• "Tat tvam asi" - "That Thou art" (Chandogya Upanishad)
• "Ayamātmā brahma" - "This Atman is Brahman" (Mandukya Upanishad)

Although there are a wide variety of philosophical positions propounded in the Upanishads, commentators since Adi Shankara have usually followed him in seeing idealist monism as the dominant force.