A rotary crusher (20) comprises first and second crushing means (22, 23) defining a crushing space (38) through which material is to be passed. The first crushing means (22) is defined by an outer surface (24) of a polygonal drum (22) which is rotated to move the outer surface (24) relative to the second crushing means (23) to alter the dimensions of the crushing space. The second crushing means (23) may be an outer surface (24) of a second polygonal drum (23), but may also be a surface (41) of an anvil (40) - see Figures 5 - 14. The crushing surfaces (24, 41) are preferably d efined by teeth (24, 41) in the form of corrugations extending the direction of rotation of the drum (22). The two sets of corrugations are relatively offset transverse to the feed direction whereby they progressively apply a crushing load to the material and also a bending load to the material transverse to the feed direction.