Abstract. Say that a separable, unital C * -algebra D C is strongly selfabsorbing if there exists an isomorphism ϕ : D → D ⊗ D such that ϕ and id D ⊗ 1 D are approximately unitarily equivalent * -homomorphisms. We study this class of algebras, which includes the Cuntz algebras O 2 , O ∞ , the UHF algebras of infinite type, the Jiang-Su algebra Z and tensor products of O ∞ with UHF algebras of infinite type. Given a strongly self-absorbing C * -algebra D we characterise when a separable C * -algebra absorbs D tensorially (i.e., is D-stable), and prove closure properties for the class of separable D-stable C * -algebras. Finally, we compute the possible K-groups and prove a number of classification results which suggest that the examples listed above are the only strongly self-absorbing C * -algebras.