This is a study on texture, decrystallization, and recrystallization in rolled and heat treated semi-crystalline polymers. Experiments were conducted using wide angle Debye-Scherrer-type X-ray diffraction. Changes in crystallinity were quantitatively monitored as a function of strain and annealing time. It was observed that crystallinity drastically drops during deformation in PET. We suggest that amorphization (decrystallization) is a deformation mechanism which acts as an alternative to crystallographic slip depending on the orientation of the nanocrystalline lamellae. Heat treatment leads to the recrystallization of amorphous PET material and to an enhancement of the original texture of the deformed crystals observed before the heat treatment. We explain this phenomenon in terms of oriented nucleation where amorphous material crystallizes alongside existing crystalline lamellae.