Subaerial pyroclastic deposits are of three genetic types: (1) fall, (2) surge, and (3) flow. Although pyroclastic flows include a wide range of volume magnitudes, only small-scale eruptions have been observed. Flows are composed of a dense, basal avalanche and an overriding cloud of entrained particles that rise by convective buoyancy. Theoretical models explain the generation of pyroclastic flows by gravitational collapse of an eruption column. The high apparent mobility of pyroclastic flows is largely due to the dissipation of kinetic and potential energy imparted at the time of eruption. The regular stratigraphic succession of fall, surge, and flow deposits is compatible with a column-collapse model. The textures, morphology, and distribution of pyroclastic flow deposits suggest emplacement as thick, dense beds that may be quasi-fluidized near their source, but which flow mainly in a laminar mode toward their distal reaches. Smaller flows may move entirely as single laminar beds. Large primary deposits may show a facies change controlled by the gradation from an inflated proximal stage into a deflated distal stage. Some deposits may undergo secondary (or tertiary) flowage following welding to produce textures and structures similar to lava flows. The various degrees of welding and secondary crystallization are principally controlled by emplacement temperature, thickness, and composition.