In his review in 1972 Woltjer  divided the evolution of supernova remnants into three different phases, the free expansion phase, the adiabatic Sedov phase and the radiative phase, when cooling of the gas becomes important. The first of these was only briefly discussed and little connection between the supernova explosion and the remnant phase was made. The reason for this is also easy to understand in view of the difficulty of determining the type of explosion even for well-known, young remnants like Cas A, Tycho, Kepler and the Crab remnant.
This situation has changed considerably during the last five years or so, mainly due to observations with new instruments like VLA, IUE and Einstein. With these, as well as large optical and infrared telescopes, the information about the supernova explosion and the early evolution of the supernova remnant has increased dramatically, and a more or less new picture of the explosion has emerged. Instead of a free expansion into a virtual vacuum, with few observational consequences, the supernova undergoes a complex interaction with its immediate surroundings, with a wealth of observational information. In this stage both the structure of the supernova ejecta and the circumstellar medium is crucial for the observational properties. Perhaps, the most interesting aspect of it is that we in this way can bridge the gap between the supernova explosion and the remnant stage.