With the aid of a high-speed cathode-ray oscillograph the surge characteristics of a variety of electrode arrangements have been studied, using surges of three different waveforms. Impulse testing technique for voltages up to 1 million volts has been revised, and results are given of the impulse spark-over of the point-plane gap and the sphere-plane gap in air and in oil, and of the spark-over of porcelain suspension insulators.