Thermocouple materials are used in temperature measurement devices and accurate temperature measurement is among the most common and vital requirements in science, engineering, and industry. This Article discusses the principles of thermocouple thermometers. It describes the types of thermocouple, including standard thermocouples, conventional thermocouples, metal-sheathed thermocouples, and nonstandard thermocouples. The Article provides information on thermocouple wires and extension wires, and presents their corresponding color codes. It discusses the different type of calibration techniques and describes the insulation and protection of thermocouples. The Article also provides information on the selection criteria for thermocouples in industrial applications, and discusses the maintenance practices of thermocouples.