Humidity sensors have potential in many fields, including health care, industrial processes, and flexible electronics. The high sensitivity of humidity sensors with high performance and stability are super important in the sensing process. To fulfill these needs, significant efforts are needed to create various types of humidity sensors. Therefore, in this chapter, a variety of materials-based humidity sensors and their sensing mechanisms such as surface acoustic wave, quartz crystal microbalance, optical fiber humidity sensors, as well as field-effect transistors are discussed. Moreover, it covers a wide range of humidity sensor applications.