Paper-based humidity sensors, conductometric and capacitive types, are currently the most common paper-based devices. This chapter analyzes the principles of their operation and the advantages of certain electrode configurations. In particular, the advantages of using interdigitated electrodes in the development of both conductometric and capacitive sensors are considered. A description of various approaches to their production is given. It has been shown that the properties of paper make it possible to use it not only as a substrate in the development of humidity and gas sensors, but also as an active humidity-sensitive material. The advantages of using metal oxides, carbon-based materials, and polymers as humidity-sensitive materials in paper-based sensors are discussed.