In order to provide representative measurements of grass, soil or earth temperatures, thermometers must be deployed in suitable locations or sites and the sensors themselves exposed in a standardised manner. This chapter sets out what those standardised conditions of site and exposure are for measurements of grass, soil or earth temperatures, 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 As a result of the UN Minamata Convention, traditional mercury-based thermometers have been progressively withdrawn from observation networks, and this chapter considers both their replacement by electronic sensors and impacts of that changeover.