The procedures adopted and the inherent assumptions made in the measurement of crystallinity of polymers by differential scanning calorimetry (DSC) are reviewed. The inherent problem in all DSC measurements is concurrent recrystallisation and melting of the polymer sample on heating to the melting point and the variation of the enthalpies of crystallisation and melting, heat capacities and degree of crystallinity with temperature, A First Law procedure is suggested which involves heating the sample between two set temperatures, T-1 and T-2. T-1 is selected by the requirement that the degree of crystallinity of the sample should not change either with temperature or time. and be representative of the sample during its use. T-1 is taken to be ambient or just above the glass transition temperature. T-2 is taken to be just above the observed last trace of crystallinity. Integrating the observed specific heat difference between the sample and the completely amorphous material during these two temperature ranges determined the residual enthalpy of fusion at T-1. Problems are noted in the use of this procedure in that the specific heat of the liquid should not be arbitrarily chosen since this leads to systematic errors in the heat of crystallisation. The degrees of crystallinity of metallocene polyethylene (m-PE) and polyethylene terephthalate (PET) measured by this procedure have been compared with values measured by density, determined at room temperature.