Book excerpt: Relative humidity is a term used to describe the amount of water vapour that exists in a gaseous mixture of air and water vapour. The relative humidity of an air-water mixture is defined as the ratio of the partial pressure of water vapour in the mixture to the saturated vapour pressure of water at a prescribed temperature. This book presents current research in the study of relative humidity, including humidity-sensitive and non-ohmic tin dioxide ceramics; optics of airborne particles under elevated humidity conditions; a review of relative humidity management and its environmental effects; relative humidity effects in a spa; and relative humidity modelled by a functional network.