X-ray and nuclear magnetic resonance (n.m.r.) measurements of crystallinity have been made over a wide temperature range on oriented and unoriented samples of polyethylene terephthalate. The results show that, although in general terms, no correlation exists between the two methods, above the glass transition a considerable proportion of the non-crystalline regions achieve molecular mobility. The n.m.r. method gives higher values of crystallinity than the X-ray method. This is apparently due to some of the non-crystalline material being immobilized owing to entanglements, and the presence of surrounding crystallites for this material will make a contribution to the broad component of the n.m.r. signal which is used as a measure of the crystalline fraction of the polymer.