The breakdown and sustaining mechanism in Te48As30Si12Ge10 alloy glass switching, using a planar electrode structure, has been investigated with the aid of a scanning electron microscope. This geometry permits direct observation of the conducting filament in the 1 μ gap samples studied. The switching and sustaining mechanism involves high temperatures in the filament, well beyond the melting temperature of the glass. A simple model is suggested for the switching which will allow sufficient energy to enter the filament and melt the glass, switching it to a high conductivity state in the subnanosecond times observed. It is suggested that avalanche breakdown initiates the predominantly thermal process.