A critical analysis of different theoretical models of overall crystallization kinetics accounting for non-steady-state nucleation effects as well as for size-dependent cluster growth is given. The overall crystallization kinetics of polyethylene terephthalate (unfilled and also containing titanium dioxide) is studied near the melting temperature T m , and in the vicinity of the glass transition temperature T g , under conditions of sporadic and athermal nucleation, respectively, by using differential scanning calorimetry (DSC) and optical microscopy. A standard method is proposed for analyzing the degree of transformation vs. time curves in order to determine the kinetic parameters of the process from DSC measurements. The temperature dependence of the induction times τ i n d shows that, in the vicinity of the glass transition temperature, they are determined by non-steady-state nucleation kinetics. In this way, it is shown that the more general, transient formulation of the classical theory of phase formation is also inherent to polymer crystallization kinetics.