Radhakrishnan's The Principal Upanishads begins with a 129-page introduction, with the following 19 section headers: General Influence; The Term 'Upaniṣad'; Number, Date and Authorship; The Upaniṣads as the Vedānta; Relation to the Vedas: The Ṛg Veda; The Yajur, the Sāma and the Atharva Vedas; The Brāhmanas; The Āranyakas; The Upaniṣads; Ultimate Reality: Brahman; Ultimate Reality: Ātman; Brahman as Ātman; The Status of the World and the Doctrine of Māyā and Avidyā; The Individual Self; Knowledge and Ignorance; Ethics; Karma and Rebirth; Life Eternal; Religion.

147–938) contains Sanskrit originals (in a romanized transliteration, rather than in Devanagari), plus verse-by-verse translations and commentaries on the following Upanishads, in this order: 1.

[1] The reviewer stated that "The Principal Upanishads"... have now been nicely translated by Sir Sarvepalli Radhakrishnan, Asia's foremost contemporary philosopher, a man as well-versed in Jewish and Christian theology as he is in the cults and culture of the East.... His book includes... a strikingly clear commentary explaining their spirit as well as their literal meaning.

[5] Journal of Bible and Religion referred to the book as "Another solid work by India's greatest living philosopher.... Radhakrishnan has selected the eighteen most important [Upanishads]" (p. 152[2]).

Philosophy stated that "The Western world was in fact already well provided with translations and critical editions," but that "the value of Radhakrishnan's version... will surely be found to lie in his commentary and the long introductory essay on the teaching of the Upanisads, for therein the Western scholar is given the interpretation of these basic documents of Hinduism reached by one of the finest minds of contemporary India after long years of study both of his people's traditional philosophy and of the thought of the West" (pp. 71–72[3]).

Editions include: The book has also been translated into other languages besides English (e.g., Hindi, Delhi: Rajapala, 1981, OCLC 19410015).