The Upanishads are a collection of philosophical texts which form the theoretical basis for the Hindu religion. They are also known as Vedanta, the end of the Veda. In the purest sense, they are not Sruti (revealed truths) but rather commentaries which explain the essence of the veda (revealed knowledge). The Upanishads are found mostly in the concluding part of the Brahmanas and in the Aranyakas.
All Upanishads have been passed down in oral tradition. More than 200 Upanishads are known, of which the first dozen or so are the oldest and most important and are referred to as the principal or main (mukhya) Upanishads. With the Bhagavad Gita and the Brahmasutra (known collectively as the Prasthanatrayi), the mukhya Upanishads provide a foundation for several later schools of Indian philosophy (vedanta), among them, two influential monistic schools of Hinduism. Historians believe the chief Upanishads were composed over a wide period ranging from the Pre-Buddhist period to the early centuries though minor Upanishads were still being composed in the medieval and early modern period. However, there has been considerable debate among authorities about the exact dating of individual Upanishads.
“The Upanishads were collectively considered amongst the 100 Most Influential Books Ever Written” as described by the well-known british poet Martin Seymour-Smith. Their significance has been recognized by writers and scholars such as Schopenhauer, Emerson and Thoreau, among others. Scholars also note similarity between the doctrine of Upanishads and those of Plato and Kant. (Wikipedia)
THE 11 PRINCIPAL UPANISHADS