In the context of global warming and the energy transition, two essential questions arise: how to cool environments without major environmental impact and how to produce heat efficiently without combustion. These questions reveal a reversal of the energy paradigm that has prevailed since the Industrial Revolution, when the challenge was to produce work from heat. Reverse cycle thermal machines (refrigeration systems, heat pumps and thermofridges), operating in reverse of the thermomechanical conversion motor cycle, have a major role to play in answering these questions, which are at the heart of the energy challenges that humanity will have to face in the coming decades. This book first presents a state of the art on these systems, whose operating principle is sometimes old, but whose performance analysis and optimization have sometimes been neglected. Emerging technologies, which will certainly find their place in the future energy panorama, are also discussed.