In very clean metals, it is now possible to observe the onset of hydrodynamic electron flow in metals.These interaction-dominated transport regimes are new solvable  models of transport describing very different phenomenology from the conventional theory of transport.   I will describe this new approach to transport physics, and present simple quantitative models for the crossover between conventional impurity-limited transport, and hydrodynamic collision-dominated transport in Fermi liquids.  I will show that hydrodynamic fluctuations modify the impurity scattering rate, providing new mechanisms for electron-limited T^2 resistivity in low density Fermi liquids.   I will also describe hydrodynamic effects on transport in metals such as graphene as they approach a charge neutrality point, and outline experimental observations of these effects.