Abstract: A prefabricated, modular building construction system uses three basic, proportioned panels, comprising rectangular wall and roof panels and triangular gable panels. The panels are secured together with simple fasteners inserted through aligned openings in adjacent edges of the panels and turned to lock them in place. The gable panels are shaped as right triangles, with the angular disposition of the hypotenuse determining the pitch of the roof and the length thereof having a specific relationship with respect to the length of the roof panels. The lengths of the other two sides also have a particular relationship to the length and width of the wall panels. Thus, the panels may be arranged in any predetermined number and size to define buildings of various sizes and shapes. Each of the panels comprises channel shaped frame members surrounding a rigid insulating core with protective and/or decorative surface layers or sheets laminated on both sides.