Abstract: A building system for the construction of walls, floors, roofs, paths and roads employs prefabricated blocks having compound shapes which are such that at least a majority thereof each exhibit projections or recesses arranged to co-operate interkeyingly with the projections or rcesses of other blocks of the system, whereby the blocks can be assembled without the essential use of mortar or other intervening binding material. The blocks may be substantially T- or Z-shaped, having a hollow formation and being flat-laid, or disposed upright, in horizontal courses in vertical walls. The hollow interiors of the blocks may be filled with strengthening material or heat-and sound-insulating material or reinforcing bars may extend through aligned hollow interiors in the superposed courses. Compound-shaped corner and junction blocks are employed, where required.