A few years ago, a real break-through happened in observational astronomy: the un derstanding of the effect of atmospheric turbulence on the structure of stellar images, and of ways to overcome this dramatic degradation. This opened a route to diffraction-limited observations with large telescopes in the optical domain. Soon, the first applications of this new technique led to some outstanding astrophysical results, both at visible and infrared wavelengths. Yet, the potential of interferometric observations is not fully foreseeable as the first long-baseline arrays of large optical telescopes are being built or cOIIllnissioned right now. In this respect a comparison with the evolution of radio-astronomy is tempting. From a situation where, in spite of the construction of giant antennas, low angular resolution was prevailing, the introduction of long baseline and very long baseline interferometry and the rapid mastering of sophisticated image reconstruction techniques, have brought on a nearly routine basis high dynamic range images with milliarcseconds resolution. This, of course, has completely changed our views of the radio sky."

For many astronomers, Adaptive Optics is something like a dream coming true. Sinee 1609 and the first observations of celestial bodies performed with the help of an optieal teleseope, astronomers have always fighted to improve the 'resolving power' of their instruments. For a long time, engineers have trimmed the optieal quality of the teleseopes, until they finally reaehed the barrier set by the atmospherie turbulence, a few seconds of are. At that point, the intrinsic quality of the site beeame a major issue to establish new observatories with modern telescopes, and astronomers started to desert the urban skies and to migrate toward mountains and deserts. This quest has been sueeessful and a few privileged sites, where the average natural 'seeing' is close to 0. 5," are now hosting clusters of giant telescopes of the 4 m and soon 10 m class. Yet, this atmospherie limit corresponds in the visible wavelength range to the diffraetion limit of a 20 em telescope only. The loss was severe: a faetor 20 in angular and several hundred in peak energy eoncentration, i. e. in deteetivity of resolution very faint objeets. In the beginning of the seventies, two doors half opened to provide a way out of this dead-end. First, the technique of speckle interferometry (and its various related developments) has allowed to restore the diffraetion limit of large telescopes at visible and infrared wavelengths (see, e. g.

The question of the existence of other worlds and other living beings has been present in the human quest for knowledge since as far as Epicurus. For centuries this question belonged to the fields of philosophy and theology. The theoretical problem of the formation of the Solar System, and hence of other planetary systems, was tackled only during the 18th century, while the first observational attempts for a detection started less than one hundred years ago. Direct observation of an extra-solar planetary system is an extraordinarily difficult problem: extra-solar planets are at huge distances, are incredibly faint and are overwhelmed by the bright light of their own stars. With virtually no observational insight to test their models, theoreticians have remained for decades in a difficult position to make substantial progress. Yet, the field of stellar formation has provided since the 1980s both the the oretical and observational evidences for the formation of discs at the stage of star birth and for debris materials orbiting the very young stellar systems. It was tempting to consider that these left-overs might indeed later agglomerate into planetary systems more or less similar to ours. Then came observational evidences for planets outside the Solar System."

A few years ago, a real break-through happened in observational astronomy: the un derstanding of the effect of atmospheric turbulence on the structure of stellar images, and of ways to overcome this dramatic degradation. This opened a route to diffraction-limited observations with large telescopes in the optical domain. Soon, the first applications of this new technique led to some outstanding astrophysical results, both at visible and infrared wavelengths. Yet, the potential of interferometric observations is not fully foreseeable as the first long-baseline arrays of large optical telescopes are being built or cOIIllnissioned right now. In this respect a comparison with the evolution of radio-astronomy is tempting. From a situation where, in spite of the construction of giant antennas, low angular resolution was prevailing, the introduction of long baseline and very long baseline interferometry and the rapid mastering of sophisticated image reconstruction techniques, have brought on a nearly routine basis high dynamic range images with milliarcseconds resolution. This, of course, has completely changed our views of the radio sky."

For many astronomers, Adaptive Optics is something like a dream coming true. Sinee 1609 and the first observations of celestial bodies performed with the help of an optieal teleseope, astronomers have always fighted to improve the 'resolving power' of their instruments. For a long time, engineers have trimmed the optieal quality of the teleseopes, until they finally reaehed the barrier set by the atmospherie turbulence, a few seconds of are. At that point, the intrinsic quality of the site beeame a major issue to establish new observatories with modern telescopes, and astronomers started to desert the urban skies and to migrate toward mountains and deserts. This quest has been sueeessful and a few privileged sites, where the average natural 'seeing' is close to 0. 5," are now hosting clusters of giant telescopes of the 4 m and soon 10 m class. Yet, this atmospherie limit corresponds in the visible wavelength range to the diffraetion limit of a 20 em telescope only. The loss was severe: a faetor 20 in angular and several hundred in peak energy eoncentration, i. e. in deteetivity of resolution very faint objeets. In the beginning of the seventies, two doors half opened to provide a way out of this dead-end. First, the technique of speckle interferometry (and its various related developments) has allowed to restore the diffraetion limit of large telescopes at visible and infrared wavelengths (see, e. g.

The question of the existence of other worlds and other living beings has been present in the human quest for knowledge since as far as Epicurus. For centuries this question belonged to the fields of philosophy and theology. The theoretical problem of the formation of the Solar System, and hence of other planetary systems, was tackled only during the 18th century, while the first observational attempts for a detection started less than one hundred years ago. Direct observation of an extra-solar planetary system is an extraordinarily difficult problem: extra-solar planets are at huge distances, are incredibly faint and are overwhelmed by the bright light of their own stars. With virtually no observational insight to test their models, theoreticians have remained for decades in a difficult position to make substantial progress. Yet, the field of stellar formation has provided since the 1980s both the the oretical and observational evidences for the formation of discs at the stage of star birth and for debris materials orbiting the very young stellar systems. It was tempting to consider that these left-overs might indeed later agglomerate into planetary systems more or less similar to ours. Then came observational evidences for planets outside the Solar System.