Collisions between relativistic heavy nuclei have been studied in fluid dynamic models. Viscosity and heat conduction do not generate much additional entropy but tend to drive the system from local to global thermal equilibrium. Pion production is a sensitive measure of the partitioning of energy among pion degrees of freedom, local temperature and global collective fluid flow. Experiments now show that nuclei are not as transparent at Bevalac energies as cascade calculations tend to suggest. Preliminary calculations in a two-fluid model indicate that for central collisions between uranium nuclei at a CM energy of 5 GeV per nucleon the central cores rebound, much as in the old Landau picture.