Abstract: Precast Building Construction System A building construction system uses precast concrete panels (10) interconnected in an array to provide both floor and ceiling of a multi-unit building. Each panel has a reinforced precast concrete slab having an upper surface (12) and downstands (18, 20) providing a continuous enclosure beneath the floor surface. Grooved voids (44, 58) are preformed into the upper and lower surface of the downstands for receiving connectors (54, 56, 57 or 60, 62, 66 and 50, 52) for connecting adjacent panels. Preformed recesses (19) are provided at the edges of the panel for connecting the panels together and creating grouted shear keys (59).