Siddha Healers in Tamil Nadu trace their practice to a lineage of Tamil ascetics (siddhars). Although no historical information about the siddhars is available, their authorship is attributed to vast bodies of knowledge, including healing practices, alchemy, yoga, and philosophy. According to Tamil mythological traditions, these ascetics gained supernatural powers through their yogic practice and devotions to the Hindu god Shiva. Since pre-colonial times, siddha medical practitioners have cited these powers of the siddhars, and the siddha authorship of their system, to justify the effectiveness of their procedures and medicines. While this continues to be true, the introduction of Western medical techniques, based on a notion of scientific rationality that denies the possibility of supernatural phenomena, has significantly altered the ways in which Tamil medical practitioners and patients conceptualize the body, illness, diagnosis, and treatment. Siddha practitioners have had to respond to these changes with various strategies to retain their authority. In my dissertation, I will study the historical development since the seventeenth century of the ways in which siddha healers have argued for their legitimacy as healers, taking particular note of the influence of allopathy (biomedicine). My work will also address more general issues of global significance -- what is the impact of Western-derived notions of science on religious beliefs in supernatural phenomena in non-Western cultures?