The European Southern Observatory (ESO) has immediate plans for the construction of a ‘New Technology Telescope’ (NTT) of about 3–3·5 m aperture and is studying a project for a ‘Very Large Telescope’ (VLT) of 16m equivalent aperture. Some characteristics of these telescopes are discussed, particularly the possibilities of greatly improved optical performance through active optics,closed-loopcontrol. In this connection the problem is approached from the physical origins of the stable or quasi-stable optical aberration function which, apart from high frequency seeing, sets the limits of optical quality of telescopes in practice.