# Hydrodynamics Hydrodynamics is a special situation of out-of-equilibrium physics. It is *the* EFT for macroscopic fluctuations on top of a (locally) equilibrium *macro-state*. For a hydrodynamic description to work, it is important that there is a separation of scales between the microscopic details and the macroscopic perturbations. At the small scale, things should be fast; at the large scale, things should be slow. Fast degrees of freedom will decay before they can affect the large scale dynamics, and the slow degrees of freedom are those that are protected by globals symmetries (i.e. conserved charges) which cannot decay locally. The conserved charges are conservation equations therefore become the only things that matter at large scales, which is called the hydrodynamic regime.