We observed the dynamics of Al2Cu precipitates, voids, and hillocks under electromigration (EM) stress (200–250° C, 1–2 MA/cm2) in Al-2wt%Cu lines using in-situ and ex-situ side-view TEM. The sub-micron wide (0.5 to 0.8 μm) bamboo Al-Cu lines were on a TiN underlayer in a drift velocity measurement structure. Large precipitates near the cathode ends of the lines shrank under EM, leaving voids before all the Cu was depleted from the cathode ends. We attribute the void formation at the cathode end to the stram induced by the shrinking precipitates in neighboring Al grams. Precipitates downstream of a sufficient Al source (eroding precipitate or Al grain) eroded without adjacent void growth. Size and location of precipitates were thus important in determining whether voids formed or not. Size and location of precipitates at the anode end, however, did not necessarily affect hillock growth. Hillocks grew near the anode end of the lines even with so-called blocking grain precipitates upstream in some cases.