Abstract: A building panel (1) has an upper edge (4) adapted to interlock with a lower edge (6) of a vertically adjacent substantially identical panel (1). The lower edge (6) of the panel (1) is adapted to interlock with a vertically adjacent substantially identical panel (1) or base (16). Side edges (8, 9) of the panel (1) are substantially planar to permit abutment of a side edge (8) of one panel (1) against a side edge (9) of a horizontally adjacent substantially identical panel (1). A continuous channel (11) extends the length of each side edge (8, 9) and a further continuous channel (10) extends along the upper edge (4). To construct a wall, a plurality of panels (1) are placed horizontally adjacent to each other to form a lower layer (15). A cementitious mix is poured onto the panels (1) to fill the channels (10, 11). Alternatively, structural members can be placed into the channels (10, 11).