Dynamics of relativistic interacting gases: from a kinetic to a fluid description

Starting from a microscopic approach, we develop a covariant formalism to describe a set of interacting gases. We model the collision term entering the Boltzmann equation for a class of interactions and then integrate this equation to obtain an effective macroscopic description. This formalism will be useful in studying the cosmic microwave background nonperturbatively in inhomogeneous cosmologies. It should also be useful for the study of the dynamics of the early universe and can be applied. if one considers fluids of galaxies, to the study of structure formation.