The potential of stereo observations by imaging atmospheric Cerenkov telescopes is currently being explored by ground based gamma-ray observatories (HEGRA, WHIPPLE, TELESCOPE ARRAY). Several motivations are given for detecting air showers simultaneously with two or more telescopes. A more effective background suppression by a complete measurement of the shower parameters might lead to a higher sensitivity to detect gamma-ray sources. The expansion of the energy coverage to lower energies by triggering several telescopes in coincidence and combining the information of the individual detectors could fill the observational gap between 30 GeV and 200 GeV. Also the energy resolution of current atmospheric Cerenkov detectors could be significantly improved with multi-telescope installations. The Whipple collaboration has pursued the stereo approach by building a second telescope of 11m diameter to carry out measurements in conjunction with the 10m telescope. First measurements indicate the detection of the Crab Nebula and Markarian 421 in the stereo mode. The stereoscopic view of the 10m/11m detector allows, for most of the coincident events, an unambiguous reconstruction of the arrival direction and therefore a determination of the source direction. This is demonstrated with data from the Crab Nebula and Markarian 421. Implications for future arrays, especially the VERITAS array, are discussed.