A system of structural blocks comprised of longitudinally extending members having vertical inner and outer faces. The basic structural blocks have webs centered on and projecting away from the inner faces of the blocks and extending from top to bottom of the blocks along one-half their longitudinal extent. Tongue and groove projections and recesses on the inner faces and on the webs mate with corresponding projections and recesses on identical blocks, whereby a plurality of such blocks may be longitudinally, transversely and vertically interlocked together. Corner, jamb, pilaster, beam, junction and web blocks are provided, together with a corresponding series of half height blocks, to interlock with the basic blocks and with each other so as to enable construction of a wide variety of buildings or structures.