A building system is described comprising paired studs (20) for attachment to inside surfaces of facing sheets (10, 11) to provide panel formwork for concrete walls. The studs comprise a head (21) and outwardly extending spaced flanges (22, 23) with inwardly directed teeth (26) and lips (24, 25). Spacer elements (30) are engaged with the recess between the flanges and may be formed from off-cuts of sheet material or as specifically formed elements. The latter are formed from hard plastic and in a preferred embodiment (19) are formed as a rectangular box which can be orientated in alternate directions. The system may include end elements (40), internal top and bottom plates (51), internal corners (90), external corners (100) and a shadow line junction between panels. The invention extends to panels formed with the above system and buildings formed from the panels.