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 have exerted an important influence on the rest of Hindu philosophy and were collectively considered one of the 100 Most Influential Books Ever Written by the British poet Martin Seymour-Smith. — Excerpted from Wikipedia, the free encyclopedia.