Carbon dioxide (R-744) is one of the naturally occurring compounds collectively known as “natural refrigerants.” It is nonflammable and nontoxic, with no known carcinogenic, mutagenic, or other toxic effects, and no dangerous products of combustion. Using carbon dioxide in refrigerating systems can be considered a form of carbon capture, with a potential beneficial effect on climate change. It has no adverse local environmental effects. In recent years, CO2 has once again become a refrigerant of great interest. However, high-pressure CO2 systems [e.g., 490.8 psia (3.4 MPa) at a saturation temperature of 30°F (–1°C), or 969.6 psia (6.7 MPa) at 80°F (26.7°C)] present some challenges for containment and safety. Applications System Design System Safety Piping Heat Exchangers and Vessels Compressors for CO2 Refrigeration Systems Lubricants Evaporators Defrost Installation, Start-up, and Commissioning