Epact is an electronic catalogue of medieval and renaissance scientific instruments from four European museums: the Museum of the History of Science, Oxford, the Istituto e Museo di Storia della Scienza, Florence, the British Museum, London, and the Museum Boerhaave, Leiden. Together, these museums house the finest collections of early scientific instruments in the world. Each instrument in the catalogue is described with the aid of one or more photographs and two levels of text: an overview text providing a short account of the most notable features of the instrument and a detailed text giving more technical and scholarly information.