Heat has traditionally been used as a specific tracer in groundwater systems. New temperature measurement techniques, as DTS, allow the diversification of these approaches. This can help improve the understanding of groundwater systems (particularly river-aquifer interactions) and the characterization of hydraulic and transport properties. Heat transfer processes in groundwater are described and the corresponding equations are introduced step-by-step. Dimensionless numbers can help assess which processes are the most important in a given situation. Low enthalpy shallow geothermal systems are described by highlighting the influences of hydrogeological conditions on their performances and environmental impacts. Additional emphasis is given to open-loop shallow geothermal systems, as they imply groundwater pumping and reinjections.