Highly filled solid-liquid pastes pose particular challenges in processing as they exhibit a range of phenomena which compromise extrudate properties as well as complicating theological measurement and constitutive modelling. Examples discussed here include wall slip, surface fracture and liquid phase migration during extrusion. Like most particulate systems, linking macro-scale behaviour to particle level structure is a real challenge, but essential if formulation is to develop from current modes based on empiricism and laboratory testing. Recent progress in understanding these phenomena is discussed, and indications of how some of these phenomena can be exploited for product advantage are given. (c) 2005 Elsevier Ltd. All rights reserved.