A helmet (10) comprising (i) an outer shell (12); (ii) an inner pad (14) configured to deform resiliently to cushion the head of a user; and (iii) a solid open-cell foam material (18) wherein the outer shell is spaced apart from the inner pad by the solid open-cell foam material and wherein the solid open-cell foam material is arranged to deform irreversibly in the event of an impact which applies a pressure greater than a selected threshold pressure to absorb the energy of the impact.