A security screen comprising a frame having an outer edge which defines the outer perimeter of the frame and further having an inner edge (12) which surrounds an open portion of the frame, the outer edge of the frame being dimensioned such that in use the security screen is receivable across an opening to close the opening, a perforate sheet element (29) received in the open portion of the frame to close the open portion, the edge of the perforate sheet element being formed with a lip (31) extending from one face of the perforate sheet element, the inner edge of the frame formed with a recess (13) having an entry extending across at least a portion of the width of the inner edge, the recess extending along the inner edge, the recess having two opposed side faces (15, 16) extending from the inner edge, one side face (16) of the recess having a zone (23) located inward of the inner edge and extending laterally from the recess, the edge of the perforate sheet element received within the recess such that the lip is located within the zone and the one face of the perforate sheet element adjacent the lip overlies the one side face, a locking member (33) fixed within the recess between the other face of the perforate sheet element and the other side face of the recess to close the space between the perforate sheet element and the other side face.