In order to provide representative measurements of air temperature, thermometers must be deployed in suitable locations or sites and the sensors themselves exposed to the weather conditions they are intended to measure in a standardised manner. This chapter sets out what those standardised conditions of site and exposure are for measurements of air temperature, following the guidelines laid down by the World Meteorological Organization in the so-called CIMO guide (Commission for Instruments and Methods of Observation). As a result of the UN Minamata Convention, traditional mercury-based thermometers have been progressively withdrawn from observation networks, and this chapter considers both their replacement by electronic sensors and impacts of that changeover. Alternative methods to measuring air temperature in the traditional Stevenson screen or Cotton Region Shelter, particularly aspirated temperature measurements, are also covered in detail.