We give formulae for the Chen-Ruan orbifold cohomology for the orbifolds given by a Bianchi group acting on complex hyperbolic 3-space. The Bianchi groups are the arithmetic groups PSL 2 (O), where O is the ring of integers in an imaginary quadratic number field. The underlying real orbifolds which help us in our study, given by the action of a Bianchi group on real hyperbolic 3-space (which is a model for its classifying space for proper actions), have applications in physics.We then prove that, for any such orbifold, its Chen-Ruan orbifold cohomology ring is isomorphic to the usual cohomology ring of any crepant resolution of its coarse moduli space. By vanishing of the quantum corrections, we show that this result fits in with Ruan's Cohomological Crepant Resolution Conjecture. 55N32, Orbifold cohomology arXiv:1109.5923v4 [math.KT] 24 Aug 2018Let Γ be a discrete group acting properly discontinuously, hence with finite stabilizers, by bi-holomorphisms on a complex manifold Y . For any element g ∈ Γ, denote by C Γ (g) the centralizer of g in Γ.Denote by Y g the subspace of Y consisting of the fixed points of g.Definition 1 ([11]) Let T ⊂ Γ be a set of representatives of the conjugacy classes of elements of finite order in Γ. Then the Chen-Ruan orbifold cohomology vector space of [Y/Γ] is:The grading on this vector space is reviewed in Equation (1) below.This definition is slightly different from, but equivalent to, the original one in [11]. We can verify this fact using arguments analogous to those used by Fantechi and Göttsche [16] in the case of a finite group Γ acting on Y . The additional argument needed when considering some element g in Γ of infinite order is the following. As the action of Γ on Y is properly discontinuous, g does not admit any fixed point in Y . Thus, H * Y g /C Γ (g); Q = H * ∅; Q = 0. For another proof, see [1].