A number of in-situ experiments, aimed at investigating geological, hydrogeological and hydromechanical aspects of HDR reservoir development were carried out at the Fjallbacka test site in western Sweden between 1984 and 1995. By means of hydraulic stimulations, a roughly horizontal reservoir that connects two 500 m deep wells was created. An open-loop circulation test performed between the wells gave a flow recovery of about 50%. All major fluid injections have resulted in microseismic activity due to shear failures. A cross-hole seismic survey of the inflated reservoir provided some additional evidence of the distribution of hydraulically active fractures.