We have reviewed humidity sensors based on various materials for both relative and absolute humidity, including ceramic, semiconducting, and polymer materials. In the majority of publications, there are few papers dealing with absolute humidity sensors, which have extensive applications in industry. We reviewed extensively absolute humidity sensors in this article, which is unique comparing with other reviews of humidity sensors. The electrical properties of humidity sensors such as sensitivity, response time, and stability have been described in details for various materials and a considerable part of the review is focused on the sensing mechanisms. In addition, preparation and characterization of sensing materials are also described. For absolute humidity sensors, mirror based dew-point sensors and solid-state AlO moisture sensors have been described. As the major problem in AlO moisture sensors, long-term instability, has been solved, AlO moisture sensors may have promising future in industry.

Humidity sensors have gained increasing applications in industrial processing and environmental control. For manufacturing highly sophisticated integrated circuits in
semiconductor industry, humidity or moisture levels are constantly monitored in wafer processing. There are many domestic applications, such as intelligent control of
the living environment in buildings, cooking control for microwave ovens, and intelligent control of laundry etc.

In automobile industry, humidity sensors are used in rear-window defoggers and motor assembly lines. In medical field, humidity sensors are used in respiratory equipment, sterilizers, incubators, pharmaceutical processing, and biological products. In agriculture, humidity sensors are used for green-house air-conditioning, plantation protection (dew prevention), soil moisture monitoring, and cereal storage. In general industry, humidity sensors are used for humidity control in chemical gas purification, dryers, ovens, film desiccation, paper and textile production, and food processing.