An elastic-plastic finite element method, based on the Prandtl-Reuss equations of plastic flow and involving equivalent stresses and strains, is used to study boudinage structure. Our choice of data for the simulations was guided by published stress-strain curves for marble (matrix) and quartzite (boudin), the essential parameters being yield stress and rock 'hardness' (defined by the slope of the stress-strain curve). All models assume an initial fracture and slight separation and therefore only simulate post-fracture behaviour. The simulations suggest that boudin shape is determined by boudin hardness; maximum stresses are concentrated in the corners which therefore shows the most shape modification. Matrix hardness determines the amount of boudin separation. Direct comparison with natural examples is restricted to boudins suffering no significant pre-fracture plastic deformation (i.e. rectangular- and barrel-shaped boudins), although other types are likely to have the characteristics of barrel and pinch-and-swell styles. The simulations do not consider the nature and timing of boudin-defining fractures but these are important in determining the style of boudinage which ultimately develops. Some mechanical problems associated with the infilling of inter-boudin gaps by ductile rock matrix are discussed and two models proposed. The first, based on yielding fracture mechanics, is used to explain boudins with wedge-shaped (or otherwise nonmatching) ends. The second, a hydraulic model, is proposed to account for gaps between rectangular boudins that are filled by ductile rock matrix.