This chapter considers several important aspects of crystallinity in polymers. A polycrystalline sample comprises many little crystals perhaps microns in size, with approximately random relative orientations. The world's most popular synthetic polymer, in terms of volume produced per year, is polyethylene; polyethylene can crystallize. Polymers are extended -dimensional objects in the crystalline state, and the overall direction of the backbone corresponds to one axis of the unit cell. Suppose a polymer spherulite grew by wrapping each chain tightly around and around the surface like a ball of string. A molten polymer is a random jumble of intertwined chains, whereas the crystal has long sections of chains fully extended and closely packed in parallel with one another. The determination of the space group and the full unit cell structure of a polymer crystal, including bond angles, bond lengths, and interchain distances, is the first goal of polymer crystallography.