Thermodynamically, polymer crystallization is a first–order transition that involves overcoming an energy barrier. Building a molecular kinetic model that links this macroscopic concept with experimental observations has been and still remains a difficult issue. It requires a physical picture that can show how a three–dimensionally random linear macromolecule is converted to a chain–folded crystalline state despite the loss of entropy in the process. There are a number of dynamic molecular pathways during polymer crystallization, and previous analytical models have used a ‘mean–field’ approach. In polymer crystallization, every macromolecule has to go through several selection processes on different length– and time–scales. In this article, we try to identify these selection processes and lay down some basic principles of polymer crystallization. Experimental observations on stem configurations, helical conformations, crystal structures, fold lengths, global macromolecular conformations and lamellar single–crystal morphologies have been used as probes to identify these selection processes.