Thermal stress induced voiding provides an effective nucleation mechanism for the electromigration induced damage and failure of narrow, passivated metal lines. Small voids are trapped at grain and phase boundaries, where they grow under a current. Growth rates are determined by the local flux divergencies and the current induced stress distribution. After reaching a critical size, some voids begin to migrate and coalesce, eventually leading to line failure. A model is outlined, which is capable of explaining a large number of experimental observations and offers a basis for the prediction of failure statistics.