The control of crystallization is important at all processing stages of the PET bottle industry, from the manufacture of bottle resins to the fabrication of preforms and bottles. In this work, we sought to evaluate critically three methods of crystallinity measurement. We have used density, Differential Scanning Calorimetry (DSC), and Modulated Differential Scanrun ' g Calorimetry (MDSC) to study the crystallinity of PET chips, preforms, and bottles. The accuracy, precision, and general validity of each technique and the problems of interpretation are discussed.