The molecular characteristics which determine whether a long‐chain polymer will crystallize at all—and if so, how readily—are considered. The criterion for crystallization is geometrical regularity of molecular structure; but absolute regularity is not essential, for certain copolymers, as well as certain homopolymers with geometrically irregular structures, do crystallize; the requirement is that the different units which indiscriminately occupy corresponding positions in the structure must be not too different in size and shape (as in monomeric ``mixed crystals''); an approximation to regularity is sufficient.It is unlikely that quite regular molecules can be incapable of crystallizing; when such substances are reported as noncrystalline, it is more likely that they crystallize only with difficulty, and the right conditions have not been found. Reluctance to crystallize is associated with a small interval between relaxation transition (``second‐order transition'') and melting points; this interval appears to be influenced by the general shape of the molecule (though shape is not the only factor). The temperature range within which crystallization proceeds at an appreciable rate may be still further restricted if the chain repeat distance of the molecules is long; and this same factor may make crystallization at any temperature very slow.