The Navier-Stokes equations are paradigmatic equations describing hydrodynamics of an interacting system with microscopic interactions encoded in transport coefficients. An extremely important problem of non-equilibrium statistical physics is to show how Navier-Stokes equations arise from microscopic dynamics of particles. In my talk, I will present a partial solution to this problem known under the name of Chapman-Enskog theory. In the first part, I will revisit the results for classical gases governed by the Boltzmann equation. In the second part, I will show how this framework can be generalised to a new class of systems, which are weakly perturbed integrable quantum gases.