  The sensitivity of charge, heat, or momentum transport to the sample geometry
is a hallmark of viscous electron flow. Therefore, hydrodynamic electronics
requires the detailed understanding of electron flow in finite geometries. The
solution of the corresponding generalized Navier-Stokes equations depends
sensitively on the nature of boundary conditions. The latter are generally
characterized by a slip length $\zeta$ with extreme cases being no-slip
$\left(\zeta\rightarrow0\right)$ and no-stress
$\left(\zeta\rightarrow\infty\right)$ conditions. We develop a kinetic theory
that determines the temperature dependent slip length at a rough interface for
Dirac liquids, e.g. graphene, and for Fermi liquids. For strongly disordered
edges that scatter electrons in a fully diffuse way, we find that the slip
length is of the order of the momentum conserving mean free path $l_{ee}$ that
determines the electron viscosity. For boundaries with nearly specular
scattering $\zeta$ is parametrically large compared to $l_{ee}$. Since for all
quantum fluids $l_{ee}$ diverges as $T\rightarrow0$, the ultimate
low-temperature flow is always in the no-stress regime. Only at intermediate
$T$ and for sufficiently large sample sizes can the slip lengths be short
enough such that no-slip conditions are appropriate. We discuss numerical
examples for several experimentally investigated systems.
