The term ‘humidity’ refers to the amount of water vapour in the air. The physics of water vapour is one of the main components of the atmospheric heat engine which produces ‘weather’ and as a result, humidity measurements are an essential requirement for operational meteorological analysis and forecasting, for climate studies, hydrology, agriculture and many other areas of human activity and comfort. In the meteorological context, the terms relative humidity (RH) and dew point (Td) are most often used in specifying atmospheric water vapour content. This chapter sets out how humidity measurements are made, following guidelines laid down by the World Meteorological Organization in the so-called CIMO guide (Commission for Instruments and Methods of Observation), including those from chilled mirror hygrometers, a dry and wet bulb psychrometer and electronic humidity sensors. Limitations of existing methods in some circumstances, such as air temperatures below freezing point, are covered.