Abstract: A modular building system for residential and light commercial construction is disclosed in which the exterior walls and , optionally, the floor, ceiling, roof and interior walls of the structure comprise a series of like prefabricated rectangular or square panels and subdivisions thereof. Interposed between adjacent panels are relatively thin metal fins somewhat wider than the thickness of the panels so that portions of the fins project beyond the surface of the panels. Within the panels are a plurality of horizontal passages, preferably uniformly spaced, which align with holes in the fins and like passages of adjacent panels. Rigid load transferring members, some or all of which are hollow, are snugly fitted into holes in the fins and extend partially into and snugly fit the aligned passages of the panel on each side of the fin.