The body of expanding gas ejected at a speed of about 10,000 km s^-1 by a supernova explosion, observed as a diffuse gaseous nebula, often with a shell-like structure. Supernova remnants are generally powerful radio sources. The evolution of the SNR can be divided into different phases according to the dominant physical processes. Simplified models are made for the first stages, to get an idea of typical time scales, expansion velocities, and sizes. The three main phases are: 1) the free expansion phase, 2) the Sedov-Taylor phase, and 3) the snowplow phase.