Typical systems of many particles in strong interaction have extremely complex behaviours which are hard to study in detail. But when the system is very large, simplicity resurfaces: just a few degrees of freedom are relevant, which follow new, simple laws. Understanding what the emergent behaviours are from the underlying microscopic interactions is one of the foremost problems in modern science. There’s one very powerful set of ideas and tools at our disposal: hydrodynamics. Although the Navier-Stokes and related equations have been studied for a very long time, we are now starting to uncover the full potential of the fundamental principles of hydrodynamics. I will provide an overview and summarise the important questions in this field, emphasising the special phenomenology of quasi-one-dimensional quantum and classical fluids realisable in modern experiments.