Heat and groundwater flow are analogous. Measurement of the change in heat flow can be used to indicate groundwater movement. In this Chapter, the physical basis of heat flow measurements is introduced with examples from heat measurements in streams where groundwater is recharging or discharging. Geothermal heat flow and thermal imagery are described with examples from remote sensing and field applications. Field measurements of heat are described using both loggers and fibre-optic equipment. The thermal properties of sediments and the general heat-flow equation are presented and examples given of the use of heat as a tracer in the unsaturated and saturated zones.