This lecture is a short introduction to Tantra as a religious phenomenon related not only to the Sanskritic élite and its textual production (such as, e.g., the highly philosophical Kashmiri Śivaism and its non-dualistic view of the universe) but also to the cross-cultural dialectic between Brahmanism and non-mainstream religions, such as tribal and folk traditions. After an analysis of the proposed definitions of Tantra in the history of the studies, the characteristics of Tantra will be point out in order to explain the problematic identification of Tantra as an univocal category because the extreme fluidity of its elements. Hence, also a differentiation between left-hand and right-hand or soft-core and hard-core Tantra emerges as a flawed attempt of classification. Finally, the controversial issue regarding the ‘birth’ of Tantra will be discussed highlighting the possibility of a multiple origin of what we call Tantra.