Within this paper, the characteristics of aquifer thermal energy storage (ATES) for building cooling are discussed for the example of an existing ATES, which has been used for the cooling of the German Parliament Buildings for almost 10 years. Based on the analysis of measurement data, it will be shown that the studied system has reached a COP between 3.6 and 7.8 over the time period considered. Based on the results of numerical simulations it will be shown that the COP can be even increased. Choosing proper operating conditions and design parameters, such as the temperature level of the cooling network or the regeneration temperature of the ATES, the efficiency of both, the studied system and future ATES projects, can be increased remarkably. (c) 2012 Elsevier Ltd. All rights reserved.