The study of the interstellar medium is certainly one of the most recently developed astrophysical topics. Indeed, to understand the physical status of this medium, one should observe it at every wavelength, especially in radio and in the ranges which are only accessible from space such as Infra Red, X and gammas. Thanks to many new observations we now know that interstellar gas is quite inhomogeneous, since it is made from relatively cold and dense phases (interstellar clouds), surrounded by a relatively hot and tenuous medium. The summer school which took place from September 1 to 16 at the...