This paper gives a brief review on the gravity flows sourced from volcanoes on land and under water. Pyroclastic flows are supported by internal gas and the air incorporated during flowage and run out a long distance as density currents. Ash-cloud umbrella is a special case of density current and the particle fallout from the umbrella is a transition to a dilute, pyroclastic density current. Subaqueous equivalents of pyroclastic flows are supported by internal gas and/or the water incorporated during flowage and are thus interpreted as either subaqueous pyroclastic flows in the strict sense or eruption-fed density currents. Debris avalanches and lahars are also important elements of volcaniclastic gravity flows both on land and under water. These pyroclastic and volcaniclastic gravity flows are thought to transform into traction-dominated flow, particle dispersion-dominated flow (grain or granular flow), fluid escape-dominated flow, or debris flows during flowage in response to the changes mainly of flow velocity, particle concentration, and shear stress. The details of these processes still remain in debate. The role of the heat in pyroclastic density current and subaqueous eruption-fed density current is a future subject to be solved.