Abstract A significant reduction in grain size can be achieved in all types of rapid solidification processing, whether depending mainly on rapid external heat extraction or on bulk melt undercooling. The microstructural observations are reviewed for melt-spun ribbon for highly undercooled melts. The origin of the refinement in these cases is analysed and is shown not to be due to copious homogeneous nucleation, which may, however, operate in refinement by devitrification.