Aquifer thermal energy storage (ATES) systems provide a method of improving the performance of more commonly installed mono-direction groundwater heating and cooling systems. Rather than using the prevailing temperature of the abstracted groundwater, ATES systems are bidirectional, therefore allowing for the interseasonal storage of low- and higher-temperature energy. This paper provides a theoretical base for ATES and an empirical review of the performance of a typical system installed in an office building in the Netherlands. This research was carried out by engineering consultancies in the UK and the Netherlands, and a UK university. The geology and hydrogeological conditions under focus can be briefly described as a confined saturated medium sand aquifer covered by horizons of clay and fine sand. A design simulation using HSTWin, a modified version of the software package HST3D, is compared against operational data collected over a 12-month period. These data were collected following 5 years of operation. The main conclusion from the case study is that there is good agreement between the HSTWin simulation and operational findings. Furthermore, it can be inferred that active ground thermal storage strategies can offer improvements over conventional ground source energy systems.