In-situ side-view transmission electron microscopy (TEM) observations of electromigration in Al-on-TiN Lines with a drift velocity measurement structure have been carried out using an ultrahigh voltage (2 MV) electron microscope. Thick chips as-diced from silicon substrates served as TEM samples. The observations revealed the dynamic behavior of electromigration-induced voids and hillocks during forward and reverse current feeding through the Al lines. The results include vertical growth of voids bounded by faceted Al, refilling of voids, void growth in a hillock upon current reversal, and whisker growth.