The Upanishads are Hindu scriptures that constitute the core teachings of Vedanta. They do not belong to any particular period of Sanskrit literature: the oldest, such as the Brhadaranyaka and Chandogya Upanishads, date to the late Brahmana period (around the middle of the first millennium BCE), while the latest were composed in the medieval and early modern period. The Upanishads realize monist ideas, some of which were hinted at in the earlier texts, and they have exerted an important influence on the rest of Hindu and Indian philosophy.

The word 'Upanishad' in Sanskrit language means upa (near), ni (down) and shad (to sit), which can be summed up as 'to sit down near' (and receive instructions). The teacher and student or a spiritual master and his disciples setting were not always prevalent in strict sense. In some cases, it was husband answering questions about immortality to his wife or a teenage boy was being taught by Yama (God of Death). The teachers, in some cases were women sages and the seekers of inspiration were kings. Besides dialogues, Upanishads also contain narrations, similes, metaphors, illustrations and symbolism.

In the quietude of caves and monasteries or ashrams situated on the banks of holy Ganges, this mystic knowledge was exchanged for centuries. It can be said that Upanishads are collection of writings representing oral transmission of such knowledge.

Most of the Upanishads are either commentary on or are an extension of four Vedas and in most cases constitute Vedanta (the end or an ultimate part of Veda). The characteristics of the Upanishads are their universality and the total absence of any dogmatism. Upanishads elaborate upon highest metaphysical state, beyond which is the realm of Silence.

Upanishads are considered as the backbone of Hinduism. The thoughts expressed inside Upanishadic cluster forms the core of Indian philosophy. One can find doctrines of Karma (action), Yoga (union), Punarjanma (rebirth), moksha (liberation), atma (soul) and brahman (super soul), inside these scriptures. Upanishads also gives valuable insight into Hindu belief system behind the creation of universe and the reasons behind its sustenance.

According to Historians, Upanishads were composed between year 800-400 B.C. Experts differ on total number of Upanishads, but most agree on 108. Major among them are Chhandogya & Kena (Sam Veda); Aitareya & Kaushitaki (Rig Veda); Katha, Taittiriya, Brihadaranyaka, Svetasvatara, Isa & Prasna (Yajur Veda); Mundaka & Mandukya (Atharva Veda). From various elaboration found inside Upanishadic verses, it can be said that, sages like Yagnavalkya, Uddalaka, Aitareya, Pippalada, Sanat Kumar, Shwetaketu, Shandilya, Manu and even Maharshi Narada disseminated Upanishadic knowledge and thus, can be said to be their authors.

Upanishads are acknowledged as pinnacle of human wisdom. None other scripture of that time can claim to contain such sublime and noble thoughts as found inside Upanishads. Written almost at the time of the dawn of civilization, Upanishads continues to evoke tremendous interest even today, among literates of both, East as well as West.

Philosophy of 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.