Transmission electron microscopy of 2.1 mu m wide Al-4 wt.% Cu interconnects during in-situ electromigration stressing shows microstructural evolution and the development of damage, including failure. Voids which develop early stop growing and are not fatal. Hillocking is associated with Al-Cu precipitates; voiding which leads to failure is associated with copper depletion. Void initiation is at the upstream end of inclined grain boundaries crossing the lines, which have a near-bamboo microstructure. Heating events occur as damage develops, arising from further voiding or from stress build-up. Open-circuit failure can occur when the proximity of grain boundaries impairs the stress-driven healing. A schematic model for open-circuit failure in bamboo lines is proposed. The relative advantages of in-situ and conventional testing are discussed.