I will demonstrate that a class of solutions to Einstein’s equations with a negative cosmological constant in $d+1$ dimensions are in one-to-one correspondence with solutions of the equations of hydrodynamics (relativistic generalizations of the Navier Stokes equations) in $d$ dimensions. This construction relates the classic area increase theorem of black holes physics to the existence of a positive divergence entropy current in hydrodynamics. The fluid gravity correspondence has proved useful—it has led to the discovery of new, previously overlooked terms in the equations of relativistic hydrodynamics for charged fluids and superfluids. It has also spurred an effort to better understand the equations of relativistic hydrodynamics as an effective field theory.