Abstract:  Efficient turbulent entrainment causes otherwise dense volcanic jets to rise high into the atmosphere as buoyant plumes. Classical models based on laboratory experiments conducted with particle-free fluids suggest that the inflow of air is 10–15% of the axial velocity, leading to predictions for the height of the plume and, in turn, the composition, structure and stability of the resulting umbrella clouds. In this talk I will summarize recent progress from laboratory experiments aimed at investigating how relatively dense, inertial particles can influence the basic mechanics of turbulent entrainment and mixing, as well as processes governing sedimentation and re-entrainment from the jet, and sedimentation from the cloud. This talk will be an informal tour of problems on which we have made progress and an open discussion of phenomena that are currently partly or completely elusive.