Tensile tests performed on extruded films of amorphous and semi-crystalline PEEK revealed close behaviours, suggesting similar deformation mechanisms. The initial deformation was elastic and homogeneous in the materials. Beyond the yield point, deformation became inhomogeneous, leading to the formation and propagation of shear bands within the material. Shear bands are more numerous and well-defined in amorphous PEEK, suggesting a better ability to localise the deformation. The observed angle of 54.7 relative to the tensile direction can be explained by a continuity criterion for displacement between the two phases under plane stress conditions. These shear bands eventually merged and formed a neck, which propagated along the sample. The deformation in the necked region is higher in semicrystalline PEEK, which is representative of the intrinsic behaviour of the material. Once the neck had propagated along the sample, deformation became homogeneous again. Microstructural analyses performed by SAXS and WAXS showed that, for both materials, the microstructure was significantly altered upon yielding, with chains becoming highly aligned along the tensile direction in the neck. While initially amorphous PEEK remained amorphous up to the ultimate strength, semi-crystalline PEEK appeared to undergo partial amorphisation upon yielding. The nature of the newly formed phase is still to be determined. The amorphous sample undergoes significant creep in the necking region during neck propagation. This effectively shifts the observed onset of strain-hardening towards higher strain-stress values.