During the last 10 years, the general concept of using the subsurface for the storage of energy has been increasingly developed in Sweden. Among the many alternatives, aquifers are one of the most promising options. Potentially they can be used both for seasonal and short-term energy storage at temperatures ranging from 2°C to more than 100°C.So far, Sweden has had only experimental experience with storage of high-temperatures (>50°C). However, there are several low temperature systems (<25°C) that have been operated for more than two years. In this paper, these ATES plants are overviewed and some general technical problems are identified, as well as measures that have been taken to solve these problems.The range of problems includes the aquifer system itself, but also problems related to the total system and how it is operated. Further, the economics are briefly described, as well as environmental aspects.The ATES concept still has some weaknesses that need improvement, but commercialization of the technology should be quite close.