The recent progress in cost and reliability of microprocessors and the interface circuits have brought more needs for every kind of reliable sensing devices. The requirements for controlling humidity as well as temperature in an ambient atmosphere have been extended to various fields such as industry, agriculture, housing and so on. Definition and its standard are first given. Next, the present status of the art and the future trend in humidity sensors and also the related instruments are reviewd. Included are smart sensors or intelligent sensors composed of silicon micro-sensors and processing circuits, and electrolyte or ceramic sensors. Finally, the relationships between solid surface and its moisture sensitive characteristics, that is, chemisorption and physisorption of water vapour on the solid surface are discussed.