The thermal crystallization from the glassy state of different samples of Poly(ethyleneterephthalate) (PET) was investigated with different techniques, such as infrared and x-ray analysis and polarized optical microscopy. The different samples of PET were obtained by ageing the glassy PET films at room temperature and at 60 degrees C; the behaviour of these films was compared with that of the fresh sample, put at the crystallization temperature as soon as prepared. The temperature of crystallization was 120 degrees C. All the techniques, investigated to follow the crystallization phenomenon, show that the fresh sample crystallizes with longer times than the others. This result was correlated with the initial structural organization of the glassy amorphous films.