Water is the most essential chemical compound for humans on the earth. Water is present everywhere, in air, in soil, in rocks, in plants and an animals. In air or in other gasses water can exit in two different forms. The term moisture refers to water in liquid form that is suspended in air or gas in form of small droplets. The term humidity refers to the concentration of water vapor in air, where the water is in a gaseous phase. <br />Humidity and temperature are among the most frequently measured physical quantities in measurement science. Whereas the measurement of temperature can nowadays be done with a satisfactory accuracy, measurement of the water vapor content of gaseous atmospheres, appears much more complex.<br /> Humidity is a physical quantity that has significant importance in a number of areas ranging from life sciences to building automation. Hence, humidity control, sensing and monitoring is important in a number of areas. Humidity sensor are important devices that can be used to measure and monitor environmental humidity. Humidity sensors have the benefits of small size, high performance and easy mass production. The requirements that humidity sensors must meet in order to satisfy a wide range of applications are a good sensitivity over a wide range of both humidity and temperature, a short response time, small hysteresis, good durability, long life, resistance against contaminants, negligible temperature dependence and low cost.<br /> Humidity is important in many applications, namely in the diagnosis of pulmonary diseases, for mapping the human respiratory system by monitoring the water vapor content of exhaled breath, meteorological applications to predict likelihood of precipitation, dew or fog, sterilizers, incubators, pharmaceutical processing, biological products, humidity control in chemical gas purification, dryers, ovens, film desiccation, paper and textile production, and food processing.<br /><br />Asst. Lec. Leal Abdullah Hamza