This paper presents the results of an experimental research on the influence of prestressing on the punching shear strength of flat slabs. The tests refer to 13 geometrically identical specimens 250 mm thick that were loaded in order to investigate the influence of bending moments (due to prestressing eccentricity), normal forces and bonded tendons on the strength and behaviour of flat slabs. The tests show that the three effects have a significant influence and should be incorporated in design provisions, although some codes neglect the influence of some of these phenomena.