The equations of hydrodynamics assert the conservation of the stress
tensor and other charges once these conserved quantities are
expressed, via constitutive relations, in terms of local
thermodynamical fields. Allowed forms of the constitutive relations
are constrained by several physical principles including a local form
of the second law of thermodynamics and the requirement that
equilibrium exist under appropriate circumstances. In this talk I
describe these constraints and their inter relations, to the extent
that these have been worked out. I also discuss consequences for the
equations of higher derivative gravity via the fluid-gravity map.
