There are over one hundred Upanisads, of which, the following ten commented upon by Sankaracarya, are regarded as genuine and, therefore, most authoritative: Isa, Kena, Katha, Prasna, Mundaka, Mandukya, Taittiriya, Aitareya, Chandogya and Brhadaranyaka.
The Isa-Upanisad teaches that both knowledge (vidya) and action (avidya) are necessary for attaining immortality. The Kenopanisad lays down that Brahman alone is the power that makes the senses function. The Kathopanisad contains the famous dialogue between Naciketa and Yama. The Prasnopanisad contains Pippalada's answers to philosophical questions put by his six disciples. The Mundakopanisad classified knowledge into Para and Apara and discusses the nature and the means of meditation upon Brahman, its result and, Brahmanirvana. The Mandukyopanisad describes, in course of explaining omkara, the three stages of soul, known as Jagarita, Svapna and Sususpti, as well as the fourth stage of Advaita. The Taittiriyopanisad deals with certain Dhyanas and instructions as means to the realisation of Brahman. The Aitareyopanisad discusses cosmogony, rebirth and immortality. The Chandogyopanisad discusses, inter alia, the significance of the mystic syllable om, meaning of Gayatri, the problem of being and non-being, appearance and reality, individual and universal spirit, subtlety and immanence of Atman, self and reality, etc. And, lastly, the Brhadaranyakopanisad contains some interesting parables and accounts of philosophical importance, ritualistico-philosophical description of the cosmos, poetico-physiological description of the self etc.