[en] Tuff is of interest for use as an isolation medium for high heat producing wastes because it provides highly sorptive minerals and suitable thermomechanical properties. Also, tuff is widespread in areas that offer long and deep groundwater flow paths. The occurrence and geologic/hydrologic setting of tuff are discussed. The properties of the rock are discussed and compared with other isolation media. The favorable and unfavorable aspects are presented. Also, unresolved issues are discussed along with the investigative program for addressing these issues