A protective helmet is described comprising: an outer layer (1); an inner layer (5) for contact with a head of a wearer; and an intermediate layer (3, 4) comprising an anisotropic cellular material comprising cells having cell walls, the anisotropic cellular material having a relatively low resistance against deformation resulting from tangential forces on the helmet. The anisotropic material can be a foam or honeycomb material. The foam is preferably a closed cell foam. The helmet allows tangential impacts to the helmet which cause less rotational acceleration or deceleration of the head of the wearer compared to helmets using isotropic foams while still absorbing a significant amount of rotational energy.