In this thesis we study the formation of nonequilibrium structures in open self-gravitating systems, such as the fragmented interstellar medium (ISM). Our interest is particulary focused on the role of gravity. Thus, in a first project numerical simulations of self-gravitating galactic disks are carried out. We find that the competing gravitational and dissipative processes produce persistent patterns formed by transient, filamentary structures. In a second project, we numerically simulate spherical N-body systems and compare them with theoretical models. We find inconsistencies in the interval of negative heat capacity, substantiating the importance of microscopic physics and the lack of consistent theoretical tools for the discription of self-gravitating gas. Finally, we carry out observations of the ISM and analyze its structure. These studies together with the numerical simulations lead to a picture of the ISM that is much more dynamical and less uniform than previously thought.