The history of the Vedanta school is well known since the time of Sankaracarya on, and its prehistory before Sankara is quite obscure. However, from the time of compilation of major Upanisads to Sankara there is a period of thousand years, and the tradition of Upanisads was not lost; there appeared many philosophers and dogmaticians, although their thoughts are not clearly known. The author has made clear the details of the pre-Sankara Vedanta philosophy, utilizing not only Sanskrit materials, but also Pali, Prakrit (Jain), as well as Tibetan and Chinese sources. In this respect this is quite a unique work. For this work the author was awarded the Imperial Prize by the Academy of Japan. Some sections of this work were already published in Indian as well as European and American journals in English. This work is a complete English translation of the entire book. The English translation was done with the financial aid by the Harvard-Yenching Institute, and the final touch was given by Mr. Trevor Leggett, the British writer, who is well-versed in Sanskrit as well as in Japanese.