Dc breakdown studies on Mylar film show that defects cause a decrease in dielectric strength with decreasing film thickness at room temperature but not at -180°C. The breakdown at defects is associated with moisture content. Using self-healing electrodes to eliminate weak areas, a dc dielectric strength of 6 Mv/cm is found, independent of film thickness. Prebreakdown current measurements indicate ionic currents which are fairly uniform throughout a sample, and field emission currents localized at incipient breakdown points. Breakdown probably occurs as a result of local heating by field emission currents.