We investigated three different modelling approaches to simulate crystallization behaviour of Ge2Sb2Te5 used in optical and electrical phase-change memories. First of these models is based on Johnson-Mehl-Avrami-Kolmogorov (JMAK) formalism to calculate the fraction of crystallized material during isothermal anneals. In the literature this model is widely used, but parameters of the model reported by different investigators varies wildly. We have showed that these discrepancies can be attributed to the ill-use of the theory. In order to overcome the restrictions put by the JMAK theory generalizations based on the classical nucleation theory have been suggested. Material parameters required by the theory, like viscosity, diffusivity, fusion enthalpy of Ge2Sb2Te5, have been deduced from published experiments. Uncertainty in the material parameters in combination with approximate expressions used by the classical nucleation theory, however, lead us to suggest a comprehensive model based on rate equations. Although it is more complicated this modelling approach has yielded more favourable and reliable results. We have discussed different simulation-experiment comparisons to illustrate the capabilities of the model.