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Professional astronomical telescopes are complex optical systems at the limit of technical feasibility. Often monolithic primary mirrors and sometimes even secondary mirrors with huge dimensions are used. Prominent examples are the two reflectors of the Large Binocular Telescope and the giant mirrors of VLT, GEMINI, and SUBARU. The performance of such precision optical components significantly depends on the physical parameters and the quality of their substrate materials. Within this paper selection criteria for mirror substrates will be discussed, thereby considering the important technical parameters as well as commercial points and aspects of project management. Qualities and limitations of classical mirror substrate materials like Zerodur, ULE, Sitall, borosilicate glass and Cervit will be evaluated and compared to new substrate materials like silicon carbide and beryllium. The different suppliers and their production processes are presented. In addition large mirrors of existing observatories and of telescopes under construction will be listed, thereby concentrating on mirrors above three meter in diameter. An outlook on material trends and on future astronomical telescopes closes this overview on the market of huge monolithic mirror substrates for optical astronomy.
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