Mass flows on hillslopes and steep channels are high gradient, subaerial mass movements of highly concentrated grain-in-fluid materials. They are generally initiated by liquefaction or erosion and bulking, and rapidly transform into non-Newtonian unsteady (surging) flows. Deposition or entrainment of material during flow may be very significant. Debris flows, most often observed in channels, consist of large clasts moving in a slurry of fine material in water, with boulders being carried at the front of large surges. Earth flows, most often observed on hillslopes, are generally large fine-grained long-lasting features that move slowly in response to rainfall and increased pore-water pressure. All kinds of mass flows can pose a significant threat to human lives and settlements.