Hydrodynamics is the universal theory describing the behavior of fluids when their spacetime variation is on scales longer than any microphysical scale in the fluid. Relativistic hydro has applications in heavy ion collisions and early Universe cosmology, and has seen a surge of interest due to heavy ion experiments and theoretical developments in AdS/CFT. I will explain what second order hydrodynamics is and why it is the minimum theory to study in the relativistic case. Then I discuss some limitations of the theory, including a new bound on how small the viscosity can be and a complication in the rigorous definition of the viscous relaxation time $tau_pi$.