**Abstract**

Temperature is one of the seven base units of the physical world, and the temperature sensors have wide applications in the lives, research, and industries. This chapter presents a brief introduction on four classic types of temperature sensors, including thermometers, thermocouples, resistance temperature detectors (RTD), and thermistors. These traditional temperature sensors have some limitations and are not suitable for dynamic measurements. To meet the demand for temperature measurement under various extreme and complex conditions, four advanced types of temperature sensors are introduced. The optical temperature sensors, including the infrared thermal imaging and laser temperature sensor, utilize the thermal radiation and are capable of measuring high-temperature objects without direct contact. The small and flexible fiber optic temperature sensors take advantage of the fact that the temperature plays a significant role in the optical transmission characteristics of the optical fiber, and it can be used in point, quasi-distributed, or distributed form. Acoustic temperature sensors measure the speed and frequency of the sound wave under different temperatures to obtain the temperature, and it is commonly used for health monitoring of complex structures. Furthermore, micro/nano temperature sensors are ideal for specific applications due to their small size, high sensitivity, and rapid response time.

**Keywords:** temperature sensor, thermocouple, optical, acoustic, micro/nano-scale
