  We show how causal relativistic Navier-Stokes equations arise from the
relativistic Boltzmann equation: the causality is preserved via a judicious
choice of the zero modes of the collision operator. A completely analogous
procedure may be used to extract causal hydrodynamics from the fluid-gravity
correspondence: again, causality of the hydrodynamic equations is preserved by
a suitable choice of zero modes of the corresponding differential operators in
the bulk. We give examples of zero modes which give rise to causal hydrodynamic
equations for non-conformal fluids with a conserved U(1) global symmetry
current.
