The electromigration performance of copper-based conductors is reviewed critically. From the available literature it appears that electromigration in copper proceeds via an interfacial diffusion path with an activation energy significantly less than that of grain boundary diffusion. As a result the advantage of copper over Al/Cu is not as great as originally anticipated, but still may be significant. Unfortunately, there are no literature data available suitable for a comparison of reliability. Because of the large relative interfacial area in state-of-the-art conductors and the substantial difference in the activation energies for interface and lattice diffusion, the "cross-over temperature" where lattice -dominated diffusion would predominate is high enough not be a factor in reliability experiments.