Pyroclastic flows are fast-moving gravity currents of hot gas and rock. They constitute some of the most devastating phenomena occurring during volcanic eruptions. I will describe experiments used to study processes that occur at the scale of individual particles in pyroclastic flows. The results of these experiments are then used to develop models for sub-grid scale processes in large-scale numerical simulations. Three questions will be addressed: (1) Where is volcanic ash generated? (2) How do pyroclastic flows travel over water? (3) What happens when a pyroclastic flow enters the ocean?