<p>CO2 is a non-flammable natural fluid with no Ozone Depletion Potential (ODP) and a negligible Global Warming Potential (GWP). Conversely, CO2 is currently responsible for over 60% of the greenhouse effect. An effective way of relieving the greenhouse effect is by recycling CO2 and using it as refrigerant, which can also be considered as a kind of CO2 capture and storage. In addition, the CO2 thermodynamic and transport properties seem to be favorable in terms of heat transfer and pressure drop, compared to other typical refrigerants. Because of these advantages, CO2 fluid has received much attention in recent years in some new energy systems, especially in CO2 trans-critical compression refrigeration thermodynamics cycle of air conditioners, and heat pumps. CO2 refrigeration and heat pump is an on-going technology, and is expected to be at the forefront of the future refrigeration and heat pump field and market. The authors explore the basic theory of thermodynamic cycles of heat pump, following with a description of the properties of CO2. Examples of CO2 application are explored, enabling the reader to follow the progression of the topic from analysis to practical usage.</p>