Sage Yajnavalkya (याज्ञवल्क्य) is a legendary sage of Vedic India, credited with the authorship of the Shatapatha Brahmana and the Brhadaranyaka Upanishad. He is also a major figure in the Upanishads

Puranic account of his life
In the Hindu view, Yajnavalkya stands distinguished both in the Shrutis and in the Smritis, and he is especially known for his unsurpassed spiritual wisdom and power. The seer of a Shukla Yajurveda (a version of Yajurveda, attributed to Yajnavalkya see White Yajurveda) from Lord Surya or the Sun God, the revealer of knowledge of Brahman to Janaka the king of Mithila and others, Yajnavalkya hails supreme among sages of sacred memory. As to his obtaining the Shukla Yajurveda (White Yajurveda) from the Sun God, there is the following story.
According to the Indian tradition, he was the son of sage Devarata and was the pupil of sage Vaishampayana. Once, Vaishampayana got angry with Yajnavalkya as the latter displayed too much sense of pride in being abler than other students. The angry teacher asked his pupil Yajnavalkya to give back all the knowledge of Yajurveda he got from him.
As per the demands of his Guru, Yajnavalkya vomited all the knowledge that he acquired from his teacher in form of eaten food. Other disciples of Vaishampayana took the form of partridge birds and consumed the vomited stuff because it was knowledge and they were very eager to receive the same.
The Sanskrit name for partridge is "Tittiri". As the Tittiri (partridge) birds ate this Veda, it is thenceforth called the Taittiriya Yajurveda. It is also known as Krishna Yajurveda or Black-yajurveda on account of it being a vomited substance. It is also called Taittiriya Samhita.
Then Yajnavalkya determined not to have any human guru thereafter. Thus he began to propitiate the Sun God, Surya. Yajnavalkya worshipped and extolled the Sun, the master of the Vedas, for the purpose of acquiring the fresh Vedic portions not known to his preceptor, Vaishampayana.
The Sun God, the glorious Lord Hari, pleased with Yajnavalkya’s penance, assumed the form of a horse and taught the sage such fresh portions of the Yajurveda as were not known to any other. This portion of the Yajurveda goes by the name of Shukla Yajurveda or White-yajurveda. It is also known as Vajasaneya Yajurveda, because it was evolved in great rapidity by Surya in the form of a horse through his manes. In Sanskrit term "Vaji" means horse. Yajnavalkya divided this Vajasaneya Yajurveda again into fifteen branches, each branch comprising hundreds of Yajus Mantras. Kanva, Madhyandina and others learnt those branches and those became popular recensions named after them.
Yajnavalkya married two wives. One was Maitreyi and the other Katyayani. Of the two, Maitreyi was a Brahmavadini ( one who is interested in the knowledge of Brahman ) . When Yajnavalkya wished to divide his property between the two wives before starting for the fourth Ashrama of his life (sanyasa) , Maitreyi asked whether she could become immortal through wealth. Yajnavalkya replied that there was no hope of immortality through wealth and that she would only become one among the many who were well-to-do on earth. On hearing this, Maitreyi requested Yajnavalkya to teach her what he considered as the best. Then Yajnavalkya elaborately described to her the sole greatness of the Absolute Self, the nature of Its existence, the way of attaining infinite knowledge and immortality, etc. This immortal conversation between Yajnavalkya and Maitreyi is recorded in the Brihadaranyaka Upanishad. The central theme of the discourse is this:
"All things are dear, not for their sake, but for the sake of the Self. This Self alone exists everywhere. It cannot be understood or known, for It alone is the Understander and the Knower. Its nature cannot be said to be positively as such. It is realised through endless denials as ‘not this, not this’. The Self is self-luminous, indestructible, unthinkable".
Wisdom of Yajnavalkya revealed to a greater extent in Brhadaranyaka Upanishad where he imparts his teachings to his wife Maitreyi and King Janaka. He also participates in a competition arranged by King Janaka about the selecting great Brhama Jnani ( knower of Brahman) and wins after defeating several learned scholars and sages. This forms a beautiful chapter filled with lot of philosohical and mystical question-answers in Brhadaranyaka Upanishad. In the end, Yajnavalkya took Vidvat Sanyasa (renunciation after the attainment of the knowledge of Brahman) and retired to the forest.
Yajnavalkya was one of the greatest sages ever known. His precepts as contained in the Upanishads (The Brhadaranyaka Upanishad) stand foremost as the crest-jewel of the highest teachings on knowledge of Brahman.

[edit] Historicity and date

The Shatapatha Brahmana and the Brhadaranyaka Upanishad are indeed dated to a similar period, roughly the 9th century BC, on linguistic and philological grounds, so that they may in principle indeed be connected to the same historical author or redactor.
Archaeoastronomical dating attempts sometimes quoted suggest dates as early as 1800 BC, but are general rejected as pseudoscience.

[edit] Works

If the ascription of the Shatapatha Brahmana and the Brhadaranyaka Upanishad is genuine, Yajnavalkya has made important contributions ot both philosophy, including the apophatic teaching of 'neti neti' , and to astronomy, describing the 95-year cycle to synchronize the motions of the sun and the moon.[citation needed]
See Shatapatha Brahmana and the Brhadaranyaka Upanishad for a detailed discussion of the content of these works