Abstract: Hydrodynamics – broadly understood as the late-time dynamics of conserved densities – is distinguished by its sparing assumption: thermalization. Its applications are therefore widespread, ranging from spin chains to galaxies. I will argue that hydrodynamics should be viewed as an effective field theory, similar to chiral perturbation theory in particle physics. This perspective allows for precision tests of thermalization in many-body systems, offers guidance and benchmarks for costly quantum simulations, and leads to surprising predictions including "diffusion cascades" and large density fluctuations in quantum Hall edges. I will end by showing how these effective field theory techniques can be used to prove a conjectured "Planckian bound" on thermalization: the equilibration time of many-body systems cannot be shorter than the quantum limit \hbar/T.