This review briefly illustrates a few fundamental concepts of cryogenic
engineering, the technological practice that allows reaching and maintaining
the low-temperature operating conditions of the superconducting devices needed
in particle accelerators. To limit the scope of the task, and not to duplicate
coverage of cryogenic engineering concepts particularly relevant to
superconducting magnets that can be found in previous CAS editions, the
overview presented in this course focuses on superconducting radio-frequency
cavities.Comment: 20 pages, contribution to the CAS - CERN Accelerator School: Course
on High Power Hadron Machines; 24 May - 2 Jun 2011, Bilbao, Spai