Collectively, the Upanishads are known as Vedanta (end of the vedas). The name has struck, because they constitute the concluding  part of the Vedas. The word 'upanishad'  is derived from a combination of  three words, namely upa+ni+sad. 'Upa' means near, 'ni' means down and 'sad' means to sit. 

Inancient India the knowledge of the Upanishads was imparted to students ofhighest merit only and that also after they spent considerable time withtheir teachers and proved their sincerity beyond doubt. Once the selectionwas done, the students were allowed to approach their teachers and receivethe secret doctrine from them directly. Since the knowledge was impartedwhen the students sat down near their teachers and listened to them, theword 'Upanishad', became vogue.

TheUpanishads played a very significant role in the evolution of ancientIndian thought.  Many schools of Hindu philosophy, sectarianmovements and even the later day religions like Buddhism and Jainismderived richly from the vast body of knowledge contained in theUpanishads. Even today if Hinduism is able to attract the attention ofmany deep thinking minds of the world, not only in India but else where, the credit goes mainly to the spiritual and philosophical depth of theUpanishads. Even the Bhagavad gita was recognized and categorized as anUpanishad in order to make it more appealing and universally acceptable asa religious scripture of par excellence.

Itis difficult to determine the age of the Upanishads. Some Upanishads, suchas Brihadaranyaka, Chandogya, Taittiriya etc., are definitely much older,while some were composed as late as the Mauryan period. The Upanishadicphilosophy evolved over a period of time, through the contribution of manyseers and self-realized souls, resulting in the complexity and variety ofthought for which they are well known today.

Thereis no definite opinion as to the number of the Upanishads. The principalupanishads are said to be 108, of which the classical upanishads are saidto be ten or twelve in number. The total number of Upanishads may exceed300.

TheUpanishads do not contain coherent and homogenous philosophical system,but many divergent ideas joined together loosely. They deal with varioussubjects such as the nature of Brahman, the ideal human conduct, thepractice of yoga, the nature of Atman, creation of the world, creation ofman, the nature of reality, the nature of true knowledge (vidya) andignorance (avidya), the nature of consciousness, the concept of karma,incarnation of soul and so on.

Theyalso deviate from the Samhita portions of the Vedas in many ways. SomeUpanishads even declare the Samhitas of the vedas as inferior knowledgeand urge the students not to waste their time in mere performance ofrituals, but to seek refuge in the Higher knowledge or the knowledge ofBrahman only.

Truly,in the field of religion and philosophy the Upanishads are the greatestcontribution of India to the world heritage. It is unfortunate that manypeople in the world, including many Hindus, are largely ignorant of thesesacred scriptures. There is no exaggeration when we say that even acursory study of the Upanishads is bound to change our thinking and waysof living.