The analysis procedure of heat capacity for polymers using DSC, as well as crucial tips for the measurement procedure and the advantages of analyzing DSC traces using heat capacity, are summarized. Heat capacities of polymers can be calculated based on group vibrations and skeletal vibrations linked to the Tarasov function, which has been established by B. Wunderlich et al. as the ATHAS (Advanced Thermal Analysis System) scheme. The ATHAS data bank contains equilibrium information on heat capacities and transition parameters for over 200 polymers and polymer-related materials. The ATHAS schemes for polydioxanone, polytetrafluoroethylene, and polyethylene terephthalate are described. The method for accurately determining the crystallinity of polymer is explained on the basis of the ATHAS data bank. The availability of heat capacity proved to be of great importance for the analysis of metastable states such as rigid amorphous and conformational disordered crystal states of polymers.