In this chapter, the „elds where organic Rankine cycle (ORC) applications are implemented and heat transfer procedures take place were summarised. The ORC was thermodynamically analysed under both subcritical and supercritical parameters. Concerning the utilisation of a heat source, the choice of the ideal cycle was investigated, and the advantages of an analysis based on the triangular cycle over the Carnot cycle were presented. As a result, the system ef„ciency was de„ned and selected as the optimisation criterion for the selection of the appropriate organic Šuid and the determination of the operational parameters of the cycle, regarding the most ef„cient utilisation of a speci„c heat source. Having analysed and de„ned the earlier parameters, the possible advantages of the supercritical ORC over the subcritical one were highlighted. However, serious concerns were raised about the heat transfer mechanisms under supercritical parameters that could result in reduced heat transfer coef„cients and consequently very large and expensive heat exchangers. Therefore, the calculation procedure of the heat transfer coef„cients and the respective results were presented, providing a useful tool for the comparison of the two cycles as well as for the design and the dimensioning of heat exchangers under supercritical parameters.