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.