The late-time, long-distance behaviour of gravitational perturbations of planar AdS black holes is controlled by hydrodynamic quasinormal modes. In the dual CFT these are modes of the conserved currents which control the infra-red dynamics. While the on-shell structure of these modes has been understood for a long-time, we have recently begun to understand how to construct an off-shell low energy effective action, an effective hydrodynamic field theory, from holography. I will describe the general lessons learnt from the gravitational analysis, sketching out a paradigm that provides a template for the analysis of general quantum systems with dissipation and long-time memory.