Submarine turbidity currents and pyroclastic density currents, despite their contrast in carrier fluid, have much in common. Both pose a significant hazard as catastrophic flows. Both environments are challenging to directly sample during emplacement, and suffer from a relative lack of observation in the field. Their deposits demonstrate a similar set of bedform structures and lithostratigraphies. How much can be learnt and shared between the different communities? Where do comparisons usefully end, and where can paradigms be borrowed and adapted? How do recent studies where pyroclastic density currents have entered water and transitioned into turbidity currents add to our understanding of flow transitions? This session aims to bring together field, experimental and numerical studies from a number of geophysical mass flow communities to share best practice, methods, and analyses and answer a number of key challenges: (1) how can we image these flows? (2) how do the basal parts of these flows interact with the bed and can we quantify basal entrainment? (3) how do we model density-stratified or multiple-layer flows effectively, considering exchange between the layers? (4) how can we scale-up analogue models to field-scale in light of recent advances in flow monitoring? (5) what are the end-member states of density flows, how does a flow transition between these end member states and can we really infer this from lithofacies analysis? (6) can we distinguish trigger or initiating processes by deposit analysis? (7) how do flows evolve as they move away from the source, down and/or around topography, and what controls this behaviour? (8) how can we understand the processes of sedimentation from density currents and how much do deposits reveal about the flow processes?