The morphology of several semicrystalline poly(aryl ether ether ketone) (PEEK) samples with narrow molecular weight distributions and of a commercial sample of PEEK has been investigated by means of differential scanning calorimetry (DSC), wide-angle X-ray diffraction (WAXD), and small-angle X-ray scattering (SAXS). The use of different molecular weights allows us to enlarge the macroscopic crystallinity range usually attainable for the commercial PEEK grades. WAXD and DSC data are in good agreement to estimate the weight degree of crystallinity values ranging from similar to 15% for the highest molecular weight sample to similar to 55% for the lowest molecular weight sample. Most of the investigated samples exhibit the well-known double melting behavior with a low and a high melting endotherm. This behavior is interpreted in the light of a melting-recrystallization mechanism which is strongly affected by the presence of entanglements in the amorphous zones. The analysis of the SAXS data shows that, under constant annealing conditions, the crystal thickness (given by the smallest length obtained from the correlation function) remains unaffected by a change of the molecular weight while the amorphous layer is found to strongly increase with increasing chain length due to the presence of entanglements. This leads to a decrease in the linear degree of crystallinity within the lamellar stacks which is consistent with the behavior of the macroscopic degree of crystallinity obtained from WAXD, DSC, and density measurements.