[en] Over the last years, a large amount of heavy ion fusion data has been collected for energies around and well below the Coulomb barrier. As to their theoretical interpretation, the state of the art may be summarized as follows: for lighter systems, roughly Z₁Z₂ less than or equal to 80, a description of fusion as penetration through a one-dimensional, more or less standard potential barrier yields a satisfactory interpretation of the experimental data. For heavier systems such an attempt fails dramatically, underestimating the sub-barrier data by orders of magnitude. This problem is discussed. 15 references