The authorship of most Upanishads is uncertain and unknown. Radhakrishnan states, "almost all the early literature of India was anonymous, we do not know the names of the authors of the Upanishads". The various philosophical theories in the early Upanishads have been attributed to famous sages such as Yajnavalkya, Uddalaka Aruni, Shvetaketu, Shandilya, Aitareya, Balaki, Pippalada and Sanatkumara. Women, such as Maitreyi and Gargi participate in the dialogues and are credited in the early Upanishads.
There are exceptions to the anonymous tradition of the Upanishads and other Vedic literature. The Shvetashvatara Upanishad, for example, includes closing credits to sage Shvetashvatara, and he is considered the author of the Upanishad. Scholars believe that early Upanishads, were interpolated and expanded over time, because of the differences within manuscripts of the same Upanishad discovered in different parts of South Asia, differences in non-Sanskrit version of the texts that have survived, and differences within each text in terms of the meter, the style, the grammar and the structure. The texts as they exist now is believed to be the work of many authors.
Scholars are uncertain about the exact centuries in which the Upanishads were composed. The chronology of the early Upanishads is difficult to resolve, states philosopher and Sanskritist Stephen Phillips, because all opinions rest on scanty evidence and analysis of archaism, style and repetitions across texts, and are driven by assumptions about likely evolution of ideas, and presumptions about which philosophy might have influenced which other Indian philosophies. Indologist Patrick Olivelle says that "in spite of claims made by some, in reality, any dating of these documents [early Upanishads] that attempts a precision closer than a few centuries is as stable as a house of cards". Some scholars have sought to analyse similarities between Hindu Upanishads and Buddhist literature to establish chronology for the Upanishads.
Patrick Olivelle gives the following chronology for the early Upanishads:
- The Brhadaranyaka and the Chandogya are the two earliest Upanishads. They are edited texts, some of whose sources are much older than others. The two texts are pre-Buddhist; they may be placed in the 7th to 6th centuries BCE, give or take a century or so.
- The three other early prose Upanisads—Taittiriya, Aitareya, and Kausitaki come next; all are probably pre-Buddhist and can be assigned to the 6th to 5th centuries BCE.
- The Kena is the oldest of the verse Upanisads followed by probably the Katha, Isa, Svetasvatara, and Mundaka. All these Upanisads were composed probably in the last few centuries BCE.
- The two late prose Upanisads, the Prasna and the Mandukya, cannot be much older than the beginning of the common era.
Stephen Phillips places the early Upanishads in the 800 to 300 BCE range. He summarizes the current Indological opinion to be that the Brhadaranyaka, Chandogya, Isha, Taittiriya, Aitareya, Kena, Katha, Mundaka, and Prasna Upanishads are all pre-Buddhist and pre-Jain, while Svetasvatara and Mandukya overlap with the earliest Buddhist and Jain literature.
The general area of the composition of the early Upanishads was northern India, the region bounded on the west by the upper Indus valley, on the east by lower Ganges region, on the north by the Himalayan foothills, and on the south by the Vindhya mountain range. There is confidence about the early Upanishads being the product of the geographical center of ancient Brahmanism, comprising the regions of Kuru-Panchala and Kosala-Videha together with the areas immediately to the south and west of these. This region covers modern Bihar, Nepal, Uttar Pradesh, Uttarakhand, Himachal Pradesh, Haryana, eastern Rajasthan and northern Madhya Pradesh.
While significant attempts have been made recently to identify the exact locations of the individual Upanishads, the results are tentative. Witzel identifies the center of activity in the Brihadaranyaka Upanishad as the area of Videha, whose king, Janaka, features prominently in the Upanishad.
The Chandogya Upanishad was probably composed in a more Western than an Eastern location in Indian subcontinent, possibly somewhere in the western region of the Kuru-Panchala country. Compared to the Principal Upanishads, the new Upanishads recorded in the Muktikā belong to an entirely different region, probably southern India, and are considerably relatively recent. In fourth chapter of the Kaushitaki Upanishad, a location named Kashi (modern Varanasi) is mentioned.