Abstract. We find necessary and sufficient conditions on a pair of rearrangementinvariant norms, and σ, in order that the Sobolev space W m, (Ω) be compactly imbedded into the rearrangement-invariant space Lσ(Ω), where Ω is a bounded domain in R n with Lipschitz boundary and 1 ≤ m ≤ n − 1. In particular, we establish the equivalence of the compactness of the Sobolev imbedding with the compactness of a certain Hardy operator from L (0, |Ω|) into Lσ(0, |Ω|). The results are illustrated with examples in which and σ are both Orlicz norms or both Lorentz Gamma norms.1. Introduction. Sobolev spaces are one of the key elements of modern functional analysis. In applications their most important property is how they imbed into various function spaces. To be more specific, compactness of Sobolev imbeddings is useful in the theory of PDEs; indeed, it is quite indispensable when the methods of the calculus of variations are used.Among the function norms defining both the Sobolev and the imbedding spaces those of Lebesgue play a primary role, though a satisfactory description of all cases, especially the limiting ones, requires other, more delicate, norms.In this paper we characterize precisely when a Sobolev space defined by a rearrangement-invariant (r.i.) norm is compactly imbedded into a function space determined by another such norm. We will use the interpolation methods developed in our previous papers [8], [13] and [14], in which optimal (hence noncompact) imbeddings were studied.Suppose that Ω is a bounded domain in R n , n ≥ 2, having a Lipschitz boundary, written ∂Ω ∈ Lip 1 .Fix m ∈ Z + , 1 ≤ m ≤ n − 1 and let N = N (m, n) = 0≤|α|≤m 1 be the number of multiindices α = (α 1 , . . . , α k ) satisfying 0 ≤ |α|:= α 1 + · · ·+α k ≤ m.