Abstract: What is the effective description of macroscopic states in a relativistic theory? Going one step beyond thermodynamics, the simplest classical effective description is given by hydrodynamics. It is well known that relativistic dissipative fluid-dynamical equations found in classic textbooks (Weinberg, Landau & Lifshitz) predict violations of causality and non-existence of equilibrium. In this talk, I will discuss how one can make sense of dissipative relativistic hydrodynamics. Time permitting, I will also discuss why some of the long-distance, late-time predictions of classical hydrodynamics are not universal.