Abstract: A building block comprising a pair of plane walls which constitute a front and rear wall thereof, the front and rear walls being formed without connecting bridges or webs therebetween and being maintained in spaced relationship by an insulating material. In one embodiment the block is formed from two block parts, the first block part including the front wall of the block and the second block part including the rear wall of the block, the arrangement of the block parts being such that the parts are maintained spaced apart by a layer of insulating material and the shape of the block parts being such that a portion of one block part projects within the confines of the other block part.