Publisher Summary This chapter discusses special relativity. In the early twentieth century, Albert Einstein proposed the special theory of relativity, a revolutionary thesis that changed the conception of space and time. For many years before 1900, it had been understood that Newton's laws of mechanics were applicable to an inertial reference frame. It was also clear that any reference frame moving at a constant velocity relative to an inertial frame was also an inertial frame. In other words, the Newtonian laws of mechanics were assumed to be valid in any inertial frame. But the laws of electrodynamics were not as clear with respect to reference frames. Time is an abstraction conceived to order the occurrence of events. As no instant of time inherently has different properties from any other, time is homogeneous. The measurement of spatial distances may be accomplished in terms of any standard unit of length. Time is measured by a clock, a device calibrated in terms of any periodic motion sustained over a large number of cycles. Physical observations are made in terms of events, discernible that happen at a particular location in space at a particular time.