This thesis is a translation of and commentary on the Tantric Buddhist Master Amoghavajra (705-74)'s Li-clki Ching gag (Japanese: Rishukyo, Taisho: 243), the Prajparamita in 150 Verses. Whilst there are some remarks of a historical and text-critical nature, the primary concern is with the text as a religious document, in the context of the scholarship and practice of the modern Japanese Shingon Sect. The Stara occupies a central position in this sect, being an integral part in its daily worship and in the academic and practical training of its priests. The Rishukyo is extant in ten versions: a Sanskrit/Khotanese fragment (?150 verses), a Tibetan 150-verse version and six Chinese versions, one of which is a lengthy, so-called Extended Version. This last is parallelled by two Extended Versions in Tibetan, and, although an examination of the Tibetan sources lies outwith the scope of this study, the thesis sketches some of the possibilities for historical research into the Buddhist Tantric tradition in Central and East Asia which these three longer recensions open up. The Chinese versions -beginning with HsUantsang's (T.220(10))- show varying degrees of esoteric influence. This fact has significance for our understanding of Amoghavajra's version, which is a well co-ordinated ritual text. The systematic philosophical and symbolic expression of traditional Buddhist teachings which is inherent in Tantric ritual intent is the focus of this thesis. To this end, modern Japanese studies of the text have been consulted, as have the commentaries by Amoghavajra and Kaai, to which modern Shingon exponents consistently trace their ideas. The thesis thus gives a general picture of an unbroken tradition which stretches back some twelve centuries, even though the abundant commentarial literature on the Rishukyo up to the Meiji period has not been used. The section of the Bibliography dealing with works on the Rishukyo comprises a list which is considerably more comprehensive than those found elsewhere.