In this paper, the history and the recent development of polymer crystallization have been reviewed briefly. After introducing the conventional Hoffman-Lauritzen theory, the recent new experimental results, especially on X-ray scattering, have been summarized. Some new models of crystallization have been reviewed, such as Strobl's mesomorphic phase model, Olmsted's spinodal-assisted crystallization theory, and Muthukumar's molecular modeling of polymer crystallization.