Book excerpt: On November 13, 1985, catastrophic mudflows swept down the slopes of the erupting Nevado del Ruiz volcano, destroying structures in their paths. Various estimates of deaths ranged as high as 24,000 residents. Though the nature and extent of risk posed by the mudflows to local communities were well documented before the event and extensive efforts had been made to communicate this information to those at risk, the affected communities were caught largely unaware. This volume analyzes the disaster's many aspects: the extent, constitution, and behavior of the mudflows; the nature of damage to structures; the status of the area's disaster warning system; and the extent of the area's disaster preparedness, emergency response actions, and disaster relief effortsâ€"both at the time of the disaster and in the first few months following the event.