The course begins with a discussion of cryocooler applications, followed by a study of thermodynamic and heat transfer fundamentals, which are then used to explain how various types of gas-cycle cryocoolers achieve temperatures from about 2 K to 150 K. The operating principles and advantages/disadvantages of the three major recuperative cycles and the three major regenerative cycles are explained. Ralph Longsworth provides a special emphasis on the Gifford-McMahon cryocooler including its early development history, operating principles, and applications.  The course covers modeling, design approaches, fabrication techniques, and measurement methods.  Millikelvin cooling technologies that can be combined with cryocoolers for cryogen-free operation at sub-kelvin temperatures are also briefly described.