Dark, nonemissive defects form on the metal cathode in most molecular organic and polymer-based light emitting devices and eventually lead to the failure of the device. These defects have been characterized in situ using optical microscopy and ex situ using atomic force microscopy and Auger electron spectroscopy. On the basis of these observations, an electromigration mechanism for the formation of dark spot defects is proposed. The high current density required to operate polymer-based light emitting devices leads to electron-induced diffusion of the Al cathode when a short circuit forms in the emissive polymer layer. This process results in a ‘‘pileup’’ of metal at the short circuit (anode) and a surrounding circular region where the Al is depleted, appearing as a dark spot on the cathode.