The work to be described deals with the reflectivity of metals at temperatures down to 20K. A historical introduction shows how the development of electronic optical theory led to an understanding of many of the important optical properties of metals while leaving unexplained an anomalously high absorbing power. The Maxwellian phenomenological and the electronic theories are given and their limitations noted. The method chosen for the measurement of reflectivity uses a double beam system with continuous balancing and the main features of the apparatus are described. The mirror surfaces have been prepared by electrolytic polishing, by casting and by annealing. The experimental procedure is described in detail and a discussion of the sources of error is given. The variation of reflectivity with both temperature and wavelength has been measured but no absolute measurements of reflectivity were made. The d.c. resistivity of comparable samples was measured over most of the above temperature range. The reflectivity results show features which are identified with the behaviour expected as a result of a theory due to Reuter and Sondheimer. On this basis it is concluded that present theories are still inadequate to account for all the optical absolution found in metals.