The Chandogya Upanishad is one of the older, "primary" Upanishads commented upon by Shankara. It is a Mukhya Upanishad, associated with the Samaveda. It figures as number 9 in the Muktika canon of 108 Upanishads. It is part of the Chandogya Brahmana which has ten chapters. The first two chapters of the Brahmana deal with sacrifices and other forms of worships. The other eight constitute the Chandogya Upanishad.

Though there are more than one hundred Upanishads, ten are principal. These are known as Dashopanishads and known for their philosophical depth and because Adi Shankara wrote commentaries on them.

Along with Brhadaranyaka Upanishad the Chandogyopanishad is an ancient source of principal fundamentals for Vedanta philosophy. Considering number of references made to this Upanishad in Brahma sutras indicates special importance of this Upanishad in Vedantic philosophy.Important spritual practices like Dahara vidya, Shandilya vidya, etc. are its speciality.

Commentary

Of the available commentaries the oldest is the one written by Adi Shankara. Adi shankara has indicated that his commentary work is a brief book for those who want to know summary of this Upanishad. Commentator Ananda Giri

Brahmanandi Tankacharya has written a brief explanation for this upanishad and Dramidacharya wrote an elaborate and detailed commentary on the work of Brahmanandi Tankacharya. Shri Ramanuja makes many references to these two scholars in his Vedanta Sangraha and Shree Bhashya both of which are commentary works.

The first and the second chapters of this Upanishad discuss the problems of liturgy and doctrine such as the genesis and significance of Aum and the meaning and names of Saman.

First chapter consists of eighteen subchapters in which the following are described:

1. Importance and greatness of universal holy syllable OM
2. Importance and greatness of Vital Life Force or Prana, and a story to explain the holiness of Prana describing how it remained untouched by evil while all other five senses got tainted by evil.
3. Fruits of meditating on holy syllable OM

Sources

• S. Radhakrishnan, The Principal Upanishads
• Introduction by Sri Adidevananda: Chhandyogapanishads (Kannada translation)