A prefabricated building system comprising wall panels having interior and exterior surfaces with insulation positioned therebetween. The interior and exterior surfaces may be of any suitable finish and appearance, depending on the building to be constructed. Each of the panels is provided with releasable locking devices on the side and/or end edges thereof which are constructed to enable the panels to be locked to adjacent panels, sills or the like after the panels have been positioned in their desired location in the building. The releasable locking devices are actuated from the exterior of the panels so that they can be conveniently locked together or unlocked, if desired, after they have been positioned in their desired locations. Each panel comprises a frame formed of elongated channel members for strength and load bearing capability. The channel members are provided with spaced clip members which serve to support the insulation between the interior and exterior surfaces of the panel. The clip members also serve to movably support the releasable locking devices. A new and improved locking and support structure is provided for supporting prefabricated roof panels on the wall panels.