When working in spacetime the focus shifts from consideration of objects to consideration of events. An event is something that takes place, an occurrence. An event might be a collision between two objects for example. Such an event is not point-like either in space or time but we normally consider its extent in space and its duration to be small enough in the context that the event may be located in spacetime by four coordinates (x,y,z,t). A more subtle event might be "object found here". That particular event allows us to track the motion of an object in spacetime.

Events may be broken down into a series of state changes in the system involved in the event . The series of state changes comprising an event may be separate and discrete or a continuum of state changes. If the system state changes continuuously in an event it may be approximated as closely as required by considering the state at distinct instants sufficiently close together.

Just as events may be considered to be made up of state changes, state changes may consist of a number of events. The shift in viewpoint accompanies a change in the spacetime scale used to examine the event or state change. For example at a large scale, a bat striking a baseball may be an event. At a smaller scale the event consist of changes in state of a number of bat and ball atoms. At a still smaller scale each change in state of an atom may consist of a photon emission event from one atom and a photon absorption event in another atom.