Three types of instability leading to glacier surges have been suggested, namely stress instabilities, temperature instabilities, and water-film instabilities. Evidence of approximately equal volumes of ice being involved in repeated surges of the same glacier favors stress instabilities as the most likely cause. Since for a steady rate of discharge a section of a glacier undergoing extending flow tends to be of the order of thirty meters thicker than for compressive flow under otherwise similar conditions, it is suggested that surges originate when the glacier flow switches between these two modes of flow. In the accumulation zone of a glacier, a surge would be triggered off by a change from extending to compressive flow over a length of the glacier. If the volume of ice and movement velocities involved are sufficiently large this could lead to repeated surging, although a tendency to stability appears to exist. In the ablation zone, the tendency to change modes of flow would be preceded by a period during which the lower part of the glacier acts as a rigid, sliding dam and increases ice thickness upstream. In this zone, if the volume discharge of the glacier increases beyond a critical limit that depends on bedrock topography, it would initiate a surge.