A modular construction system is described. The system includes blocks (1) composed of structural slabs (4) and spacers (5), with the spacers (5) being arranged to form orthogonal and diagonal cavities (6, 7) extending within the blocks (1). When a number of blocks 1 are combined, the cavities combine to extend across blocks. Reinforcing members in the form of joists (8) can be located within the cavities, together with fastening members, to secure the construction. The principal system components may be manufactured from second-grade and/or reclaimed wood-based products and the like, to provide an inexpensive construction material. Additional cladding members or fascias can be mounted to the slabs (4), to alter aesthetic appearance or to provide functional properties such as insulation.