  This paper introduces the fundamental continuum theory governing momentum
transport in isotropic nanofluidic flows. The theory is an extension to the
classical Navier-Stokes equation, which includes coupling between translational
and rotational degrees of freedom, as well as non-local response functions that
incorporates spatial correlations. The continuum theory is compared with
molecular dynamics simulation data for both relaxation processes and fluid
flows showing excellent agreement on the nanometer length scale. We also
present practical tools to estimate when the extended theory should be used.
