This article investigates how authenticity is construed and negotiated in four different fields of reenactment practice in Denmark (Iron Age, Middle Age, World War II and Francis of Assisi). It first outlines some key theoretical positions within recent international academic debate on reenactment and living history. Taking the viewpoint of the reenactors themselves, the article explores and compares how they create, experience and negotiate authenticity in the very process of imitating and embodying pasts. It transpires that authenticity is articulated, construed and evaluated differently, according, inter alia, to whether the primarily purpose is to learn about the past or rather to learn from the past. For some reenactors, the attempt to get as close as possible to the past connects to an ideal of historical accuracy, a standard from which all replicas and performances are measured. Yet a pragmatic recognition that the past can never be recreated completely is constantly present. For other reenactors, the doing of pasts is a way of accessing experiences and values that are felt to have been lost in modernity. At the same time, however, it is all-important to them that the world they imitate is a past that actually existed and not a fictional universe.