This text is aimed at presenting the basic ingredients necessaryto understand shell-type supernova remnants, with particularemphasis on their X-ray emission.The other observational domains (radio and optical) are mentionedbut not discussed in depth.I do not discuss at all the synchrotron nebulae, such as the Crab Nebula,which are also relics of a supernova explosion, but are driven bythe remaining pulsar rather than the mechanical energy of the supernova.After a brief historical introduction, I describe the hydrodynamicconcepts used for supernova remnants. Then I describe the Sedov modelgoverning middle-aged supernova remnants, and the coronal conditionsunder which the X-ray emission is computed.Quantitative formulae are given for the Sedov model,allowing to relate X-ray observations to energy, ambient density and age.A chapter describes the initial (young) and final (old) stagesof supernova remnants.Cosmic-ray acceleration at shocks is described, as well as itsconsequences on supernova remnants. The final chapter dealswith statistical issues.