Given a Hopf algebra A, there exist various cohomology theories for thecategory of Hopf bimodules over A, introduced by M. Gerstenhaber and S.D.Schack, and by C. Ospel. We prove, when A is finite dimensional, that they areequal to the Ext functor on the module category of an associative algebraassociated to A, described by C. Cibils and M. Rosso. We also give anexpression for a cup-product in the cohomology defined by C. Ospel, and provethat it corresponds to the Yoneda product of extensions.