Metallic mirrors will be essential components of all optical spectroscopy and imaging systems for plasma diagnosis that will be used at the next-step magnetic fusion experiment, ITER. Any change of the mirror performance, in particular reflectivity, will influence the quality and reliability of detected signals. At the instigation of the ITER Design Team, a dedicated technical and experimental activity aiming at the assessment of mirror surface degradation as a result of exposure to the plasma, has been initiated on the JET tokamak. This paper provides a comprehensive overview of the mirror test programme, including design details of the mirror samples and their supports, their locations within JET and the issue of optical characterisation of the mirrors both before and after exposure. The post exposure characterisation is particularly challenging in JET as a consequence of an environment in which both tritium and beryllium are present.