This review provides a comprehensive overview of humidity sensors, their principles, mechanisms, and fabrication technologies. The article discusses various types of humidity sensors, including resistive, capacitive, and impedance-based sensors, and their applications in different fields such as agriculture, climatology, and industrial automation. It reviews the use of organic and inorganic functional materials, such as porous ceramics, polymers, and electrolytes, in humidity sensing. The review also highlights the importance of transduction mechanisms, such as ionic and electronic conduction, and the role of fabrication technologies in sensor performance. The paper emphasizes the need for improved sensor efficiency, faster response times, and better durability. It also discusses the advantages of thick film and thin film processes in sensor fabrication. The review concludes with a detailed analysis of the working principles of protonic-conduction type ceramic humidity sensors, including the mechanisms of water adsorption, proton diffusion, and hydroxide ion mobility. The paper also covers the development of polymer-based resistive humidity sensors, their advantages and limitations, and recent advancements in sensor design and fabrication techniques. The review highlights the importance of humidity sensing in various applications and the ongoing research efforts to improve sensor performance and reliability.This review provides a comprehensive overview of humidity sensors, their principles, mechanisms, and fabrication technologies. The article discusses various types of humidity sensors, including resistive, capacitive, and impedance-based sensors, and their applications in different fields such as agriculture, climatology, and industrial automation. It reviews the use of organic and inorganic functional materials, such as porous ceramics, polymers, and electrolytes, in humidity sensing. The review also highlights the importance of transduction mechanisms, such as ionic and electronic conduction, and the role of fabrication technologies in sensor performance. The paper emphasizes the need for improved sensor efficiency, faster response times, and better durability. It also discusses the advantages of thick film and thin film processes in sensor fabrication. The review concludes with a detailed analysis of the working principles of protonic-conduction type ceramic humidity sensors, including the mechanisms of water adsorption, proton diffusion, and hydroxide ion mobility. The paper also covers the development of polymer-based resistive humidity sensors, their advantages and limitations, and recent advancements in sensor design and fabrication techniques. The review highlights the importance of humidity sensing in various applications and the ongoing research efforts to improve sensor performance and reliability.