A polymer can be partially crystalline, such as polyethylene (PE), isotactic polypropene (it-pp), and poly (ethylene terephthalate) (PET), or noncrystalline, such as commercially available atactic poly (methyl methacrylate) (PMMA) and polystyrene (a-PS); partially crystalline polymers are customarily called crystalline polymers. The degree of crystallinity is an important parameter for crystalline polymers. The physical and mechanical properties of polymers are profoundly dependent on the degree of crystallinity. It is well known that the degree of crystallinity can be determined by a variety of physical methods, for example, X-ray diffraction, calorimetry, density measurements, infrared spectroscopy (IR), and nuclear magnetic resonance (NMR). Determination of degree of crystallinity by X-ray diffraction has been claimed to be inherently superior to other methods [1–4]. This review describes the concept and measurement methods of degree of crystallinity with emphasis on X-ray diffraction.