Supernovae are cosmic explosions that mark the end of massive stars and inject a significant amount of energy to the ISM. After the initial shock breakout, the ejecta sweeps through the ISM and finally merges with the ISM. This may last for more than 100000 years. This dynamic evolution of the supernova remnants can be divided into different phases according to the physical processes that dominate during the evolution. I will discuss these phases in detail and show that by applying some reasonable assumptions the typical time scales, expansion velocities and sizes characterizing these phases can be approximately extracted.