Hydraulis or Hydraulic organ is the first musical keys-bearing instrument in the world history and the predecessor of the later ecclesiastic organ. It consists of one or more sets of metallic pipes of various sizes. They are supplied with air of high and steady pressure through a hydraulic mechanism and they sound by means of special lever-keys. Hydraulis is a simple as well as a genius construction, which proves the high level of technological thought in antiquity. The inventor of hydraulis is the renowned engineer Ctesibius, an inhabitant of the third-century B.C. Alexandria. After its invention hydarulis is quickly disseminated in the Hellenistic world and later in the Roman Empire and becomes the favorite musical instrument of emperors, such as Nero. Blow devices replace progressively the hydraulic mechanism and thus the hydraulic instrument is transformed into a wine one. Hydraulis disappears from the west part of the old Roman Empire after the barbaric raids in Europe during the first Christian centuries, but is continues to survive in Byzantium as a wind musical instrument. The well-known ecclesiastical organ of the West is the evolution of the wind instrument, which was presented in 757 by the Byzantine Emperor Constantine the Copronymous to Pepine the Short, king of France and father of Charlemagne. In August 1992 the archaeologist and Professor at the University of Thessaloniki D. Pantermalis and his team discovered the upper part of a first century B.C. hydraulis during the excavations at ancient Dion. In the beginning of 1995 the European Cultural Center of Delphi, at which the author of this article presided, started a research project for the reconstruction of the Dion hydraulis with the support of the Greek Ministry of Culture and the scientific contribution of Professor Pantermalis. Head of the entire project was appointed the late Professor Marios Mavroidis and after his death the musicologist C. Stroux. Important participants in the project were P. Vlaggopoulos and G, Paraschos, the latter as builder of the reconstruction. The project was completed in June 1999 and the final result was an exact, as possible, replica of the ancient hydraulis of Dion. The reconstruction problems had to do with the hydraulic mechanism, the keys mechanism and the pipes. The solution of the fisrt two was based on the author’s interpretation of the text of Heron of Alexandria, which enabled the building of a plastically rendered model that served for the measuring of water and air pressure. Then two more models were built: the materials of the Dion hydraulis were used for the first one as well as the size, number and length of pipes of the find, while all the relevant ancient techniques were employed for making the second and final reconstruction of the ancient hydraulis.