Thus, most of the Upanishads were originally conveyed via an Oral Tradition called Sruti; which means the details were remembered and conveyed by the teacher to the student thru oral dialogues (speech). It was much later that these texts were written down under the Written Tradition called Smriti.
There are supposed to be more than 200 known Upanishads out of which 108 are recognized as being “true” Upanishads. Out of these 108, there are 13 Upanishads which are considered as the main- i.e. Primary Upanishads.
These classic Vedic scriptures are time immemorial and are meant to be read, re-read and studied to understand the true nature of our consciousness and existence.
List of Primary or Mukhya Upanishads:
1. Īśā from Yajurveda
2. Kena from Samaveda
3. Kaṭha from Yajurveda
4. Praṣna from Atharvaveda
5. Muṇḍaka from Atharvaveda
6. Māṇḍūkya from Atharvaveda
7. Taittirīya from Yajurveda
8. Aitareya from Rigveda
9. Chāndogya from Samaveda
10. Bṛhadāraṇyaka from Yajurveda
Some consider the following three also as the primary Upanishads:
• Shvetashvatara Upanishad
• Kaushitaki Upanishad
• Maitri Upanishad
These 13 Mukhya Upanishads are accepted as some of the most important scriptures of Hinduism.