Describing one of the most important practices of hathayoga (khecarimudra), the Khecarividya of Adinatha is presented here to an English-speaking readership for the first time. The author, James Mallinson, draws on thirty Sanskrit works, as well as original fieldwork amongst yogins in India who use the practice, to demonstrate how earlier tantric yogic techniques developed and mutated into the practices of hathayoga. Accompanied by an introduction and an extensively annotated translation, the work sheds light on the development of hathayoga and its practices.