In any internal combustion engine, the amount of heat rejected from the engine, and associated systems, is a result of the engine inefficiency. Successfully recovering a small proportion of this energy would therefore substantially improve the fuel economy.The Rankine Cycle system has been raising interest for its aptitude to produce systems capable of capturing part of this waste heat and regenerate it as electrical or mechanical power. By integrating these systems into existing hybrid engine environments, it has been proved that Rankine Cycle system, which is more than 150 years old, can play a major role in reducing fuel consumption. The use of such a system for waste heat recovery on a hybrid engine represents a promising compromise in transforming the thermal energy into electricity and feeding this electricity back to the vehicle drivetrain by using the in situ electrical motor system or storing it into batteries.With this aim in mind, a comprehensive background study on the existing technologies and state of the art of small-scale applications of the Rankine Cycle is carried out. For each of the main components of the desired system, a set of available solutions is proposed and their feasibility, performance and maturity are analysed. Conclusions on the optimal solutions available to use in a Rankine Cycle system for waste heat recovery on a hybrid vehicle application are discussed and justified.