This thesis explores Tibetan ideas regarding the life-span and the various life-forces that influence longevity. It presents a substantial body of ethnographic data from seventeen months of fieldwork among Tibetans in the Darjeeiing Hills, West Bengal, India (between June, 2004 and May, 2006), an area where I lived for long periods of time since 1992. The thesis supplements this ethnographic material with translations from two Tibetan medical texts (twelfth and seventeenth century CE) and a selection of astrological tables and divinatory texts that are used by Tibetans in this region today.