The object of this book is to introduce the subject of ejector in refrigerating plants in view of energy savings. A first part concerns the use of an ejector in a heat-operated system for the production of cold (refrigeration or air conditioning). The ejector refrigeration system using an environment friendly fluid allows combining two advantages: one related to the energy saving by the use of thermal energy wasted from industrial processes at average or low temperatures or a free energy source (solar energy) and the other related to the environment protection by reduction of CO2 emissions in the atmosphere. A second part consists of an original simulation program of a transcritical CO2 refrigeration system using an ejector as an expansion device in order to improve the COP of the basic transcritical CO2 system by reducing the isenthalpic expansion losses.