Extrusion-spheronisation (E-S) is a widely used technique for the manufacture of pellets with high sphericity and narrow size distribution. A dimensionless framework for describing the evolution of pellet shape with spheronisation time is presented.The experiments employed a 45 wt% microcrystalline cellulose (MCC)/water-based paste, with up to 15 wt% calcium carbonate added to represent a 'hard' active pharmaceutical ingredient (API). The bulk yield strength of the paste was measured and found to increase with increasing carbonate content, which in turn led to higher extrusion pressures, longer spheronisation times and less spherical pellets for a given set of operating parameters. The pellet aspect ratio was found to increase in a linear manner with the logarithm of spheronisation time, progressing towards an asymptotic final value. This behaviour is compared with two simple models.The dimensional analysis of the pellet rounding stage identified the paste density and bulk yield strength as scaling quantities. The model borne from the dimensional analysis was validated and was found to be applicable to a second material (MCC/lactose/water).High speed imaging was also used to examine the collision behaviour of pellets during the breakage and rounding stages in spheronisation. The results confirmed that the rounding phase was the rate-determining step. The velocities of a number of tracked pellets were also quantified, and were consistently lower than the tip speed of the rotating friction plate. Bulk yield strength velocity factor, Equation (16) Pa (s m -1 ) m  Wall shear stress velocity factor, Equation (16) Pa (s m -1 ) n  p Friction coefficient, pellet-pellet - s Friction coefficient, pellet-spheroniser surface - Density kg m -3  Y0 Bulk yield strength at zero extrudate velocity, Equation (16) Pa  Y Bulk yield strength Pa  w  Wall shear stress Pa  0 Wall shear stress at zero extrudate velocity, Equation (16) Pa  Spheroniser rotational speed s -1 Acronymns API Active pharmaceutical ingredient E-S Extrusion-spheronisation MCC Microcrystalline cellulose PEEK Polyether ether ketone PIV Particle image velocimetry