Composed between 800 BCE to the start of common era, these texts are connected to the Vedic tradition.
[1] The Principal Upanishads, which were composed probably between 600 and 300 BCE, constitute the concluding portion of the Veda.
[3][4][5] The founders of the major schools of Vedanta, viz., Adi Shankara and Madhvacharya wrote bhāṣyas (commentaries) on these ten Principal Upanishads.
In the Ramanuja lineage, one of his followers, Rangaramanuja, wrote commentaries on almost all of the Principal Upanishads around the 1600s.
[8] The Principal Upanishads are separated into three categories: prose (Taittirīya, Aitareya, Chāndogya, Bṛhadāraṇyaka), verse (Īśā, Kaṭha, Muṇḍaka), and prose (classical Sanskrit) (Māṇḍūkya).