The Kālacakratantra tradition represents the last phase of medieval Indian Buddhism. It developed in India between the end of the tenth and the beginning of the eleventh century and was soon transmitted to Tibet where it was accepted as one of the most important Vajrayāna systems. Its syncretistic and encyclopedic approach makes its literature a very interesting and rich field of study. In this lecture we will analyze the history of its early phases according to the extant sources.