Abstract: A construction block of generally rectangular form having parallel upper and lower faces between which extend, through the block, one or more wide apertures and a plurality of narrow holes; the end faces of the block have wide grooves which cooperate, in a wall structure constructed from a plurality of horizontal courses of such blocks, to provide chambers between adjacent blocks of a course, the wide apertures in the blocks of adjacent courses cooperating to form vertically extending channels, the narrow holes house locating pins for holding the blocks of adjacent courses together until they are joined by concrete cast into the said vertical channels; the said upper and lower parallel faces have central depressions which form, between adjacent courses of blocks, horizontal channels linking the vertical channels so that the concrete cast into the vertical channels can also flow into and fill the horizontal channels to form an interlinked network giving great strength to the wall.