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. Epact consists of 520 catalogue entries and a variety of supporting material. All European instruments from the four museums by makers who were active before 1600 have been entered in the catalogue. They include astrolabes, armillary spheres, sundials, quadrants, nocturnals, compendia, surveying instruments, and so on. Examples range from ordinary instruments for everyday use to more extravagant and often lavish pieces destined for the cabinets of princes. 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.