A severe rainfall event occurs once every two years in Hong Kong, causing widespread landslides which result in human casualties and much damage. It has recently been established that these landslides are caused largely by short-period, high-intensity rainfall, with nearly all failures occurring within a few hours of peak intensity. The failures are therefore attributable mainly to rapid transient variations in groundwater conditions. These findings are in contradiction to previously accepted theories on failure mechanisms, and it is therefore necessary for these to be reconsidered.A new theoretical model is now being examined in Hong Kong to explain the rapid landslide response following high-intensity rainfall. This is based on well-defined hillslope hydrological theories. Of particular significance in this context are thought to be subsurface erosion pipes and tunnels which have been found to exist in some of Hong Kong's hiilslopes, especially those carpeted with colluvium.