Theoretical approaches addressing the mechanism of polymer crystallization remain the great challenge in polymer science. Numerous different, or even conflicting, models/theories have been proposed during the past several decades. However, none of them can fully satisfy the whole community. In this Perspective, we first trace the roots of these models/theories back to the classical and nonclassical nucleation theories. The correlation between these theories and milestone theoretical works in polymer crystallization is elucidated together with their intrinsic drawbacks. Then the newly proposed two-step nucleation scenarios, with either bond-orientational order or density fluctuation as precursors, are introduced, which, in our view, may stimulate the development of polymer crystallization theory. Afterward, the peculiarities of polymer crystallization due to chain flexibility and connectivity are discussed. A personal outlook on the ultimate polymer crystallization theory is given at last, which is suggested to address the following three questions: (i) How do flexible chains transform into rigid conformational ordered segments? (ii) How do interlamellar amorphous layers form? (iii) Are polymer chains dragged by force to the growth front? Answering these questions may eventually end the tough journey for the establishment of polymer crystallization theory, though polymer crystallization never completes fully.