When crystallizing macromolecules, one needs to recognize that they are sufficiently long to connect neighboring phases. This coupling between the phases is the reason that on ordering, practically all polymers develop a globally metastable, semicrystalline structure, consisting of multiple phases with different degrees of mobility. The ordered and amorphous phases are separated by nanophases of strained segments of molecule of nanometer dimensions, consisting of rigid-amorphous fractions, RAFs, which soften, above, within, or below the often very broad melting range of the ordered phases. Thus, ordering of a melt of macromolecules causes significant changes in the remaining amorphous phases. A summary of the various phases is given in terms of their structures, molecular vibrations, and large-amplitude motion as they influence the glass and order-disorder transitions. This is followed by a description of the limits of the classical nucleation theory of crystals when applied to macromolecules. Discussed in detail are the problems in primary and secondary nucleation which arise from the presence of RAFs. Special macromolecular nucleation processes that needed to be considered are the self-nucleation on cooling from above the melting temperature or on heating from above the glass transition temperature, and the molecular nucleation causing molar mass segregation on ordering. Finally, the glass transitions of the phases of various sizes and degree of order are discussed on hand of selected, pertinent examples. Published by Elsevier B.V.