Cosmic rays appear to be universal in galaxies with star formation or nuclear activity. They play a role in the dynamics, energy balance, and stability of the diffuse gas in these systems, and are now included in many state of the art models of galaxy formation and evolution. The exchange of energy and momentum between cosmic rays and thermal gas is largely collisionless, mediated by kinetic scale plasma waves. The properties of these waves depend both on local conditions and the global structure of the thermal gas, introducing complex feedback loops that are only beginning to be probed theoretically and incorporated into models. I will discuss some recent progress in this area, focusing on the role of interstellar clouds.
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