The book introduces, edits, and translates the two earliest texts of the hathayoga tradition, the Amrtasiddhi and the Amrtasiddhimala (which survives only in Tibetan translation). Basing their study on a bilingual manuscript, an extremely rare phenomenon, the authors argue that the origins of hathayoga are found in an eclectic tantric Buddhist milieu, probably active in the second half of the 11th century CE. The texts provide fundamental and later very influential teachings on the nature of the yogic body, psycho-physical practices centered on manipulating bindu, the types of practitioners, and much more. The book is addressed primarily to scholars, but will also be of interest to students and practitioners of yoga.