Copper is becoming the metal of choice for interconnect applications in integrated circuits due to its lower electrical resistivity and higher electromigration (EM) resistance. The effects of grain size, preferred orientation, and interfacial layers, on electromigration in Cu lines are, however, not yet well understood. In this paper, we compare the EM characteristics of epitaxial Cu(111) lines with that of polycrystalline lines with a (111) texture. We also investigate the effects of Ta, TaN and TiN interfacial underlayers and Cu crystal quality on the EM response of epitaxial Cu(001) lines. EM tests were carried out on 2-μm-wide Cu lines at a constant current density between 0.02 and 3.5 MA/cm2 in an Ar ambient at 300 °C. Our results indicate that EM resistance of both (111) and (001) epitaxial lines have a higher mean time to failure (MTTF) than polycrystalline Cu lines. The presence of a TiN or TaN interfacial layer increases the MTTF in Cu(001) films, while a Ta underlayer degrades EM resistance. X-ray diffractograms indicating a smaller full-width-at-half-maximum of Cu(001) films on nitride underlayers suggest that lower defect densities in these Cu films is the likely reason for improved EM resistance. Both (111) and (100) orientations fail by formation of faceted voids.