As the refrigeration industry moves towards low-GWP refrigerants the number of acceptable refrigerants is decreasing due to new environmental constraints, but the variety of options is increasing. These "new" refrigerants include new molecules, such as the HFOs containing a carbon-carbon double bond, as well as some very "old" fluids, such as ammonia, carbon dioxide, and hydrocarbons. The HFC refrigerants, although being phased down, remain in use—especially as blend components. Here, I will review the different thermodynamic characteristics of these fluids; sometimes the differences are subtle, and sometimes they are dramatic. The transport properties also differ. Together, the thermodynamic and transport properties determine the relative performance of fluids in a refrigeration cycle. The different properties will be summarized and their impacts on cycles reviewed. The design engineer must recognize the properties of the chosen refrigerant and design the system accordingly.