The discussion in this chapter very loosely interprets the cryogenic temperature range to include temperatures from 280 K down to absolute zero, 0 K. Many aspects of thermocouple usage are the same at cryogenic temperatures as they are at higher temperatures; for example, the basic principles, as discussed in Chapter 2, the material descriptions and precautions in Chapter 3, and the measurement systems in Chapter 6, apply to both high and low temperatures. There are, however, significant differences in selection of wire size, insulations, and methods of practical usage [1, 2]. Some significant high-temperature problems such as oxidation, reduction, migration, and annealing are not present to the same degree in low-temperature applications.