This invention relates to a helmet, comprising an external shell (9) and an impact-absorbing helmet liner (10) in which the helmet liner comprises a layer provided with protruding, deformable and impact-absorbing hollow knobs (7) ditributed on an substantial part of the surface of the helmet liner as aprotective layer. The helmet is characterized in that the hollow knots (7) are arranged for deformation by crushing in case of external impact on the helmet, and that the helmet liner (10) with the hollow knots (7) distributed exhibits undercut. The helmet may also comprise an impact-absorbin and preferably resilient layer (2) which may be arranged between the external shell (9) an internal lining (11) being arranged to rest in contact with the head of a user.