(57) s7 A modular partition system comprising a plurality of partition panels (11) each composed of an interior framework of frame members (14-22) which reinforce and secure together two parallel cladding layers (12, 13) of rigid material the boundary edges of which are offset with respect to the frame members (15, 16, 20, 22) defining the boundaries of the supporting framework so as to define upper and lower channels (23, 24) and cooperating tongue (23) and groove (25) formations in the lateral edges. A partition is installed by locating a batten (28) along an original wall and a floor plate (26) having an upstanding ridge (27) in alignment therewith so that two channels (24, 25) in the lower edge and one lateral edge of the panel (11) can be engaged over the tongues thus formed to locate and interlock a panel in position. Subsequent panels can be engaged on the tongue (22) projecting from the free lateral edge of the panel (11) and the upstanding ridge (27) of the floorplate (26).