A relationship between the macroscopic toughness, the intrinsic network (entanglement and/or crosslink) density and the relative thickness of polymeric systems, is presented. Toughness of amorphous, glassy polymers is mainly determined by the strain to break, since the yield stress generally only varies between 50 and 80 MPa. It was found that the strain to break strongly depends on the absolute thickness of the specimen or, equivalently, the local thickness within the (micro) structure of the material. Only below a certain critical thickness can the intrinsic strain at break of a polymer be reached. The absolute value of this critical thickness and the intrinsic strain at break of a polymer are both determined by the network density. In this paper polystyrene (PS), a polymer that is generally considered to be very brittle, was investigated with respect to the influence of absolute thickness on its strain to break. For thin isotropic tapes of PS it was demonstrated that this critical thickness is below 1 µm. Based on experiments with macroscopically ‘thick’ PS samples (3 mm), which are made locally thin by the introduction of small, non-adhering rubbery particles (‘holes’), we could identify that the critical thickness is 0.05 µm for PS.