The mathematical treatment of light scattering by regular and random arrays of points is outlined. The diffraction patterns of two‐dimensional models are compared with patterns from ideal crystals, imperfect crystals, and paracrystalline lattices. The effects of various types of crystalline imperfections and paracrystalline order on the diffraction pattern are analyzed and examples are presented. It is shown that both natural and synthetic polymers can be treated as paracrystalline materials. The diffraction patterns of polyethylene after various thermal and mechanical treatments are discussed. A detailed model of the structure of polyethylene is presented and various features of the structure of other polymers are pointed out.