Astronomy has experienced a great boost over the last 20 years. As a consequence, the telescopes used are getting larger and larger. In the past, radio telescopes with a reflector diameter of up to 100 m were built. Now, even optical telescopes are being planned with main mirror diameters exceeding 40 m! For all these telescopes the structural system - the steel structure - and the related mechanical components - the bearings and drives - are the "backbone" for the optical elements and play a major role in the planning, construction and related overall costs. From the structural engineer's point of view, the design and construction of a telescope structure is very different from all other applications of steel structures. The design is not dominated - as it usually is - by issues of strength and stability, but mainly by issues of deformations and dynamics. The requirements for the structural system have to be developed in close cooperation with the overall telescope system engineer, who has to integrate the needs of the astronomers, the optics engineers, control engineers, etc. From the market point of view, there are only a few large telescope projects in progress worldwide, and not many structural engineers are working in this field