The behaviour of reinforced concrete slabs in fire conditions strongly depends on the support conditions and the interaction of the slabs with the surrounding structure. Previous research has shown that compressive restraint forces from the surrounding structure can increase the fire resistance of heated slabs, and fire resistance can also be considerably enhanced if tensile membrane behaviour is mobilised. This study was carried out to investigate the effects of compressive membrane action and tensile membrane action on the behaviour of reinforced concrete flat slabs in fire conditions. The investigation into compressive membrane action was performed on single span, one-way flat slabs with a non-linear finite element program, SAPIR. The slabs were subjected to the ISO fire from below. The height of the line of thrust at the supports and the axial restraint stiffness were varied to model different support conditions. The investigation on tensile membrane action was performed on unrestrained two-way flat slabs and composite slabs. The study was performed with experimental fire tests and 3D finite element analyses with SAPIR, which was used to predict the behaviour of the tested slabs and also used to model different slab configurations. The experimental fire tests were carried out for six reinforced concrete and composite steel-concrete slabs on a fire resistance furnace, each of them exposed to the ISO standard fire while carrying a live load. The SAPIR analyses of one-way flat slabs showed that their behaviour in fire conditions was very sensitive to the end support conditions and the axial restraint stiffness. The experimental tests and SAPIR analyses of two-way slabs showed that tensile membrane action significantly increased the fire resistance. The finite element predictions of the experimental fire tests with SAPIR showed good agreement. It is concluded that one-way slabs with flexural continuity at the end supports and low axial restraint have excellent fire resistance. Compressive membrane action can increase the fire resistance of pin-supported slabs if the axial restraint stiffness is very high and if the line of thrust at the supports is located near the slab soffit. The fire resistance of unrestrained simply supported concrete slabs is significantly increased if tensile membrane action can be mobilised.