How polymers crystallize? This question stands for long time and argues for many years. Over the years various conceptual models have been proposed and work out. Among them

nucleation and growth theory

which developed by Hoffman and Lauritzen

is now widely used and dominant in the data treatment. However the recent results show that the formation and growth of lamellar cyrstallites is a multi-step process passing over intermediate states. This new development and new models are summarized in this paper. The novel model developed by Prof. Strobl is favorably induced. It is proposed that the initial step is always the creation of mesomorphic layer which spontaneously thickens

up to critical value

where it solidifies through a cooperative structural transition. The transition produces a granular crystalline layer

which transforms in the last step into homogeneous lamellar crystallites.