In order to effectively meet the new sociocultural challenges that had arisen around the eve of the second millennium (first of all the Islamic invasion in the Northern India and the revival of the Brahmanical traditions), the first authors of the Kālacakra sought to establish, within a consistent framework, a new Buddhist orthodoxy and to close the Buddhist ranks around the undisputed authority of the monastic community. With a particular emphasis on doctrines, the lecture will examine the strategies adopted by the first Kālacakra masters (in particular Pundarika, Vajrapani, Vajragarbha, Naropa, Ravisrijñana and Abhayakaragupta) in order to define and characterize Buddhist Tantrism as a whole and in particular the Kalacakra school in comparison with other Buddhist and non- Buddhist traditions.