 The earliest among these were likely composed in 1st millennium BCE, while the last ones in the late medieval era.All of the Vaishnava Upanishads either directly reference and quote from the ancient Principal Upanishads or incorporate some ideas found in them; most cited texts include the Brihadaranyaka Upanishad, Chandogya Upanishad, Katha Upanishad, Isha Upanishad, Mundaka Upanishad, Taittiriya Upanishad and others.In some cases, they cite fragments from the Brahmana and Aranyaka layers of the Rigveda and the Yajurveda.