A security fence mounted to a stable base (22) comprises multiple laterally spaced fence posts (12), interconnected with horizontal rails (14), Each said fence post comprises a lower end portion (18) anchored to a foundation (20) in the stable base (22) with the remaining major portion (15) of the height thereof inclined at a mean angle of between 50° to 65° to the horizontal, the posts being inclined to a common side and extending, preferably arcuately, to a point wherein said upper terminal end portion (24) is positioned so as to preclude direct simultaneous physical access to said upper terminal end portion (24) and either said base (22) or the foot (at 26) of the inclined portion (15). Fence panelling (16) is fixed to and between said posts and extends over the height thereof. An extension (30) may be fixed to the upper terminal end portion (24) of each post to project inwardly and downwardly to the common side to which the posts incline, and panelling (32) is mounted on and extends between the extensions (30). A dual fence installation is also described which precludes scaling from both sides.