  We study the interpolation sets for the Hardy-Sobolev spaces defined on the
unit ball of ${\bf C}^n$. We begin by giving a natural extension to ${\bf C}^n$
of the condition that is known to be necessay and sufficient for interpolation
sets lying on the boundary of the unit disc. We show that under this condition
the restriction of a function in the Hardy-Sobolev space to the set always
exists, and lies in a Besov space. We then show that under the assumption that
there is an holomorphic distance function for the set, there is an extension
operator from these spaces to the Hardy-Sobolev ones.
