The Morse-Sard theorem, and the generalizations by Dubovitskiui and Federer, have numerous applications and belong to the core results of multivariate calculus for smooth mappings. In this talk we discuss extensions of these results to suitable classes of Sobolev mappings. The potential lack of differentiability of these mappings warrants new interpretations, and requires one to prove that the Sobolev mappings enjoy Luzin N type properties with respect to lower dimensional Hausdorff contents. The talk is based on joint work with Jean Bourgain (Princeton) and Mikhail Korobkov (Novosibirsk).