A full understanding of fuel cladding performance is critical to enabling the safe and flexible operation of fuel rods to high burnups in high-duty nuclear power plants (NPPs). Several European pressurized water reactors (PWRs) are by design challenging to the fuel because they achieve high burnups and operate at significant boiling duties. To verify the performance under these challenging conditions, Westinghouse PWR fuel cladding materials have been evaluated in several test programs, with rod average burnups between 14 and 73 MWd/kgU. The measurements show that the highest burnup fuel rods with Optimized ZIRLO^TM cladding have a peak measured liftoff layer (oxide plus crud) thickness of ≤44:m, whereas the reference cladding type Duplex D4, with an inner Zircaloy-4 component and an outer layer of a dilute zirconium-based alloy, had an even lower maximum liftoff of ≤32:m. The increased dimensional irradiation stability for the niobium-containing alloy Optimized ZIRLO was clearly seen by an end-of-life maximum rod growth of 12 mm, whereas the Duplex D4 had a maximum rod growth of 22 mm. Good predictions of the oxide thickness were achieved using the STAV7 fuel rod performance code. In addition, this paper includes measured and predicted hydrogen contents for three rods that were discharged from fuel assemblies at the Gösgen NPP after two, four, and five annual operating cycles, reaching rod average burnups of 37, 65, and 73 MWd/kgU, respectively. The extensive follow-up programs performed provide excellent opportunities to follow the performance of individual rods from low to very high burnups. Through comparisons with fuel rod performance calculations, the impact of the challenging conditions has been evaluated.