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Most of ITER optical diagnostics will be equipped with in-vessel metallic mirrors as plasma viewing components. These mirrors will be exposed to severe plasma environment and must withstand these conditions without change of their optical properties. This implies important research and developments on the design and manufacturing of such components. Therefore, investigations on engineering and manufacturing have been carried out on diagnostic mirrors toward the development of full-scale stainless steel and TZM (Mo-based alloy) ITER mirrors. Several-micrometer coatings of rhodium and molybdenum have been deposited on the components to ensure long-lasting of the mirrors exposed to an environment which could be dominated by charge-exchange neutral flux. Three major issues have been addressed and reported in this paper: First, investigations have been performed on the design and manufacturing of the integrated cooling system to limit the mirror optical surface deformation due to radiations from the plasma and nuclear heating. For the thermomechanical design of the mock-ups, a plasma radiation flux of 0.5 MW/m2 and a neutron head load of 7 MW/m3 have been considered. Second, the polishing capability of full-scale (109 mm in diameter) metallic mirrors has been demonstrated: The mock-up surface front error is lower than 0.1 μm root mean square, and the mirrors exhibit low roughness (Ra <; 2 nm) and low surface defects (scratch width lower than 0.02 mm) after polishing. Third, the manufacturing feasibility of thick molybdenum and rhodium coating layers deposited by magnetron sputtering has been evaluated. The objective of depositing layers up to 3-5 μm thick has been achieved on the mock-ups, with spectral reflectance reaching the theoretical values and showing high reflectivity over a large spectral range (from 400 nm to 11 μm). Finally, the test campaign of the manufactured mirrors, which is being prepared in sever- l European facilities to expose the mirrors to deuterium plasma, ELMs, neutrons, erosion, and deposition conditions, is reported.