Some time ago a proposal was made [1] for an approach to calculating the initial crystallinity for polymers using MTDSC, i.e., the crystallinity of the sample before the start of the experiment. The essence of the technique is that it attempts to estimate the contribution from the vibrational heat capacity to the total enthalpy absorbed by the sample over the temperature range where crystallization, rearrangement and melting occur. This estimate is made from the reversing signal by using interpolation to subtract any contributions to this signal from crystallisation rearrangement or melting processes. The difference between the total enthalpy and the estimate of the vibrational heat capacity contribution must then be a measure of the enthalpy of melting of any initial crystallinity. The thermodynamic basis of this approach is presented in this article together with examples of its application to both simple and complex systems. There are alternatives to this procedure and the advantages and disadvantages of the various possible approaches are discussed.