Long-burn D-T discharges in ITER will rely on spectroscopy for real-time monitoring and control of impurity production and He-ash retention, for basic machine protection and for ion temperature measurements. Data will only be reliable if the integrity of calibration of the diagnostics is preserved. This is difficult in the hostile environment inside the vacuum vessel. The optical properties of mirrors and windows may be seriously degraded due to irradiation by neutrons and g-rays, bombardment by energetic particles and deposition of eroded material. Also, radiation-induced absorption and luminescence may compromise signal transmission through quartz or fused silica windows. As sensitive components such as detectors will be located outside the biological shield, the optical systems will be distributed. Hence, maintaining alignment will be paramount when various supporting structures are moving differentially. The use of fibres can ease the problem at optical wavelengths. However, because of the long lengths used, near the machine fibres will exhibit the same effects when irradiated as quartz windows. The two tokamaks that have operated with a D-T mixture under conditions closest to those envisaged in ITER are JET and TFTR. However, in ITER a number of key parameters will be enhanced by significant factors, eg neutral particle fluxes by ~5, neutron fluxes by ~10, pulse lengths by ~100 and neutron fluences by ~10,000. On JET and TFTR, a number of methods were implemented to maintain reliable spectroscopic calibration. Several of these techniques, and other approaches, are reviewed here. They fall into two categories: those which eliminate the effects or reduce them to a tolerable level and those which employ in-situ monitoring of optical performance or compensation techniques.