I discuss the formation and growth of the medium around massive stars, especially Wolf-Rayet stars, and its impact on the evolution of the resultant shock wave when the star ends its life as a SN. The circumstellar medium is formed by mass-loss from the star during its various evolutionary stages. Using analytical calculations and numerical simulations, I study the evolution of this medium, multidimensional effects such as hydrodynamical instabilities, and the onset of turbulence. When the star explodes as a SN, the resulting shock wave will interact with this medium. I further explore the impact of the ambient medium on the expansion of the shock wave and the evolution of the remnant. Similarly, the impact on GRB afterglows and absorption spectra of GRBs is considered.