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Plans for a European Extremely Large Telescope are quite well advanced. However examination of instrument designs has thus far been directed only at covering the anticipated science requirements and has had little impact on telescope design considerations. Nevertheless, the provision of a suitable environment for instruments is a critical part of the design of all large telescopes. We illustrate this point with examples from recent experience. A Work Package, part of a proposed Design Study for a European ELT under the European Union's Framework Programme 6 (FP6), will explore this issue, while also developing designs for a scientifically credible instrument suite. For three instruments mechanical and optical design studies will be carried out in sufficient detail clearly to identify design drivers for the telescope. These are a wide-field seeing limited or ground-layer AO-corrected (GLAO) optical/NIR spectrometer, WFSPEC; an MCAO-corrected O/NIR Multi-Object Multi-field Spectrometer-Imager, MOMSI, which offers particularly daunting challenges; and a mid-infrared high-resolution AO-corrected Imager-Spectrometer instrument, MIDIR. Five instrument designs will be examined in less detail: an extreme-AO (XAO) corrected coronagraphic imager-spectrometer known as Planet Finder (the goal of which is the detection and characterization of terrestrial exo-planets); a very high resolution spectrometer, HISPEC; a high time-resolution instrument, HITRI, intended to allow photometry, polarimetry and phase-resolved spectroscopy of faint rapidly varying objects; a fast-response broad-band multi-function instrument known as GRB-catcher; and a sub-millimeter imager, SCUBA-3. A separate small study will seek innovative designs not included in the main suite. Another will initiate the program by examining the requirements of atmospheric dispersion correction (ADC) for 30 to 100-m diffraction-limited telescopes, which may require active sensing and, possibly, "adaptive" correction on atmospheric turbulence timescales. All these studies -- except that of SCUBA3 -- will require support from Adaptive Optics studies, as most instruments will be utterly dependent on AO: close communication between instrument and AO groups is essential, here and in general.
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