This paper presents a formal theory for reasoning about motion of spatial entities, in a qualitative framework. Taking over a theory intended forspatial entities, we enrich ittoachieve a theory whose intended models are spatio-temporal entities, an idea sometimes proposed by philosophers or AI authors but never fully exploited. We show what kind of properties usually assumed as desirable parts of any space-time theory are recovered from our model, thus giving a sound theoretical basis for a natural, qualitative representation of motion.