  We generalize the concept of conserving, Phi-derivable, approximations to
relativistic field theories. Treating the interaction field as a dynamical
degree of freedom, we derive the thermodynamic potential in terms of fully
dressed propagators, an approach which allows us to resolve the entropy of a
relativistic plasma into contributions from its interacting elementary
excitations. We illustrate the derivation for a hot relativistic system
governed by electromagnetic interactions.
