Stellar wind mass-loss and photo-ionization can modify the ambient medium around
massive stars during their evolution. Using numerical simulations, we discuss the
formation of the circumstellar medium (CSM) around massive stars, and the evolution
of supernova shock waves within this medium. The shock-CSM interaction heats up the
gas to X-ray emitting temperatures. The output from numerical simulations can be used
to compute the X-ray spectra and lightcurves using non-equilibrium ionization
calculations. These can be compared directly with observations, allowing us to place
strong constraints on the structure and properties of the medium and thereby on the
progenitor star, infer the abundances of the ejecta and surrounding medium to
reasonable accuracy, and explore the 3D geometry.  This is illustrated with examples
of specific supernovae.