Brihadaranyaka Upanishad is one of the oldest Upanishads and often referred to as the greatest both in substance, length, and theme. It is estimated that Brihadaranyaka Upanishad was compiled about 2.700 years ago from much older, now lost, material. Brihadaranyaka Upanishad means “Upanishad of the great forests” and it centers around the fundamentals of the Atman, Soul, or Self.