This thesis reports on the work carried out by the author on the crystallisation kinetics and optical properties of thin amorphous Ge₂Sb₂Te₅ films. This study is prompted by the application of these films in optical phase change recording media. The work is partly theoretical and partly experimental in nature. Starting from the general nucleation and growth theory, it was shown that the Johnson-Mehl-Avrami-Kolmogorov (JMAK) equations are equally valid under linear heating conditions as under isothermal annealing conditions. All peak shift methods for determining the JMAK parameters were shown to be equivalent and valid within the JMAK framework. The crystallisation kinetics of amorphous Ge₂Sb₂Te₅ films were investigated under isothermal annealing conditions. Crystallisation was found to be preceded by a temperature dependent incubation time. The JMAK parameters were determined and found to be in good agreement with the nucleation and growth parameters, which were determined independently. The optical properties of many materials can be described by three parameters: the plasma energy, Ep, an effective transition energy, Δ, and a damping constant, Γ. A theoretical model was developed that aims to calculate these parameters from the co-ordination of nearest neighbours in the structure. The model was found to work for a simplified silicon Hamiltonian and it is described how the model could be extended to include more realistic systems. Ep, Δ and Γ were determined before and after crystallisation of amorphous Ge₂Sb₂Te₅ films. From the numerical values of these parameters, it was concluded that the change in the reflectivity upon crystallisation is primarily due to the change in Δ. The change in the co-ordination of nearest neighbours upon crystallisation was investigated. The topological co-ordination was found to be very similar in both phases. The change in the optical properties is thought to be due to the change in chemical order of the nearest neighbours.