In general, a cryocooler can be developed using an object-oriented design concept applied to a prototype. There are many various demands, such as, cooling temperature, cooling capacity, reliability and cost, efficiency. On the other hand, a number of cryocoolers has already been developed. Therefore, the basic characteristics of these cryocoolers should be clarified to ascertain their adaptability to current demands. From this perspective, this fundamental cryogenic refrigerator review explains the functions of individual components for specific cryocoolers. Example cryocoolers include a GM-cooler, Stirling cooler, pulse-tube cooler and thermo-acoustic cooler.