Abstract. Let R be an isolated Gorenstein singularity with a non-commutative resolution A = End R (R ⊕ M ). In this paper, we show that the relative singularity category ∆ R (A) of A has a number of pleasant properties, such as being Hom-finite. Moreover, it determines the classical singularity category D sg (R) of Buchweitz and Orlov as a certain canonical quotient category. If R has finite CM type, which includes for example Kleinian singularities, then we show the much more surprising result that D sg (R) determines ∆ R (Aus(R)), where Aus(R) is the corresponding Auslander algebra. The proofs of these results use dg algebras, A ∞ Koszul duality, and the new concept of dg Auslander algebras, which may be of independent interest.