Introduction
Our current knowledge about the high-energy non-thermal universe in the TeV energy regime is mainly based on observations with ground-based instruments. The field of ground-based gamma-ray astronomy has been pioneered by the work of the Whipple collaboration, which successfully discovered the first source of TeV gamma-rays (the Crab Nebula) in 1989 - less than 20 years ago. Meanwhile, results from the latest generations of telescopes have revealed a sky rich with different classes of objects emitting gamma-rays in this energy regime. More than 80 TeV-sources are known so far, with energy spectra reconstructed from about 100 GeV up to almost 100 TeV. Most of these sources have been discovered during the last few years with the H.E.S.S. stereoscopic system of imaging atmospheric Cherenkov telescopes, with the MAGIC telescope, the CANGAROO telescopes and others. The H.E.S.S. survey of the central part of our galaxy has shown a large number of new sources that nicely line up with the galactic plane. Some of these cosmic particle accelerators exhibit complex structures that could be resolved for the first time. Most sources have plausible associations with known objects like pulsar wind nebulae, supernova remnants or pulsars that are also visible in other wavelength bands. However, a new class of “dark sources” has also been discovered, which have not yet been seen at other energy bands. These sources might be a clue to solving the puzzle of the origin of the galactic cosmic rays. The discovery of two TeV-gamma-ray emitting X-ray binaries by H.E.S.S. and MAGIC has opened the possibility to study the physics of very compact, solar mass objects (neutron stars or black holes) as part of these binary systems. Extragalactic sources have been detected up to distances of three billion light years. The shape of their observed gamma-ray energy spectra probes the density of the electromagnetic radiation field in the intergalactic space, which is closely related to the hotly debated history of cosmological structure formation.
The impressive physics achievements obtained with the present generation instruments has triggered the initiative of European astrophysicists to build the future ground-based gamma-ray observatory CTA. A first meeting has taken place in Berlin in May 4-5, 2006. Since beginning of 2008, the CTA consortium is performing a Design Study for the optimization of the performance of the planned observatory and to study its possible implementation.