The contributions of the hydrodynamic model to the problem of extracti
ng the equation of state are discussed with special emphasis on the mi
nimum experimental input needed and the role of viscosity. Next the pr
oblems of hydrodynamics at ultrarelativistic energies and the methods
for obtaining a reliable and accurate solution are introduced. Finally
some fundamental problems concerning the hydrodynamic equations of mo
tion in the dissipative case are indicated.