The study of the universe at very short wavelengths, in the domain of X-rays

and gamma rays, started immediately after the beginning of the space age,

with the launch in 1957 of the first artificial satellite, Sputnik 1. Three extraordinary discoveries mark the birth of high energy astrophysics: X-ray binaries (and the interesting mass exchange between normal stars and compact objects), the IntraCluster Medium (a very high temperature plasma), and Gamma Ray Bursts (huge explosions occurring frequently at cosmological distances). The final section of the chapter is devoted to a brief description of the definition of fluid models. After a brief summary of the main picture, the text describes the concept of hydrostatic equilibrium, which plays a key role in the study of the hot diffuse medium in clusters of galaxies and in diagnosing the amount and distribution of dark matter in those systems. Then the opposite regime, of rotation-dominated inviscid disks, is considered, which is at the basis of the modeling of rotation curves in galaxies. Finally, the general framework of the fluid description in the presence of viscosity, which is associated with the picture of accretion disks, is presented.