There are 108 generally accepted Upanishads, but according to different sources, the number varies upto 200. The oldest of these works dates back to 600 BC. They are later works than the Sanhitas Mantra and the Brahmanas and are a completely different category of texts. They contain a freedom of thought unknown in any of the earlier works, except the Rig-Veda. The Upanishads are more universal and can be read by all, unlike the others, which were supposed to be taught to and read by only the Brahmins.

Since the Upanishads form the concluding portion of the Vedas, they were called Vedanta or "the end of Vedas". However, the term Vedanta now refers to a school of philosophy based on the Upanishads. There are 10 principal Upanishads: The Aitareya Upanishad of the Rig-Veda. The Brihadaranyaka, Isha, Katha and Taittiriya Upanishads of the Yajur Veda.

The Chandogya and Kena Upanishads of the Sama Veda. The Prasna, Mundaka, and Mandukya Upanishads of the Atharva Veda.

It is in the Brihadaranyaka Upanishad that a typically value-based story appears: 'What the Thunder Says'. Prajapati, the All-Father, having created the three races of gods, men and demons, appointed each to their own realm - heaven, earth and the netherworld. All three begged him for advice to live by. So, to each race, Prajapati gave counsel.

To the gods (sura), he said "Damyata". Be restrained. To mankind (nara), he said "Datta". Give. To the demons (asura), he said "Dayadhvam". Be merciful. And so, forever more, when the thunder peals DA-DA-DA, his children know that it is the voice of Prajapati, the All-Father, reminding them of the key to their true selves.