The Tantras are the most esoteric of the canonical texts. The word itself, derived from a root meaning 'to spread', is applied to a variety of treatises, and affords no clue to the contents of these works. While resembling the sutras in literary form, they differ from them in dealing with ritual and yoga rather than with ethics and philosophy and in being unintelligible without the traditional commentary.
Moreover, the techniques they prescribe can be practised only when, through the rite of abhiseka or 'aspersion', the requisite spiritual power has been transmitted to the disciple by a spiritual master in the succession. How many Tantras were originally published it is impossible to say.
Standard editions of the Tibetan Kanjur contain twenty-two huge xylograph volumes of these works, to which must be added twenty-five volumes of so-called Nyingmapa Tantras. Some Tantras exist in various degrees of expansion and contraction, each set of such recensions making up a complete Tantric Cycle, the publication of which is associated with the name of a particular Siddha or 'Perfect One'.
The greater part of this enormous literature is now available only in translation, the principal collections being the Imperial Chinese Tripitaka and the Tibetan Kanjur or 'Translated Word [of the Buddha'.
Within the last hundred years, however, a number of Sanskrit Buddhist texts, both canonical and non-canonical, have come to light in Gilgit (Pakistan) and been recovered from the sands of Central Asia. While the value of the Buddhist canonical literature will always be primarily spiritual, much of it provides, at the same time, a useful corrective to any view of the social, cultural, and religious history of India derived exclusively from brahmanical sources.