Abstract: A security barrier (1) comprises four elements (2, 3, 4 and 5), each bolted to the top of a wall W. Each element comprises a generally disc-shaped plate having a central recessed portion (10) and an annular main portion (12) integrally connected to the recessed portion (10) by a shoulder (13). A support (11) for each element comprises a bolt (15) and a spacer (16), the bolt (15) extending through an aperture (18) in the centre of the element. Each plate has a toothed edge (19), all around the circumference. Alternate elements have the plates facing in opposite directions so that they are offset in plan by double the depth of the shoulder (13), and so can be interleaved. The element plates are generally parallel to the plane of the wall. With any of a variety of plate materials and thicknesses the barrier may be adapted to any of a range of security applications.