This thesis gives an account of experiments which investigate the detection of light by, and the emission of light from, planar metal-oxide-metal (M-O-M) tunnel junctions. The particular focus of attention is the mediation of these processes by surface plasmons, or surface electromagnetic waves bound to metal-dielectric interfaces, in the two processes. It describes how the coupling of incident bulk radiation to a surface plasmon supported by the junction structure may enhance the response of the device when used as a photodetector. This idea is then extended to cover other electromagnetic resonances supported by the junction system in different operating configurations. There is a brief departure from M-O-M devices to consider how a metal-semiconductor Schottky barrier diode may also have its photoresponse enhanced in a similar manner by coupling to a surface plasma wave localised at the metal-vacuum interface before returning to M-O-M devices to show that, in addition to their use as discrete detectors, they may also be used as integrated detectors of guided radiation. Attention is then turned onto the reverse process of light emission from M-O-M tunnel junctions. When these devices are 'rough' or are corrugated in some manner and pass a current they emit broadband light with an upper frequency cut-off determined by the applied bias, hv= e<SUP>V<SUB>bias</SUB></SUP>. This light emission process is mediated by the surface plasmons of the structure, of which there are three in the energy range considered. Experimental results on the light emission from residually rough and deliberately roughened junctions are reported. In particular, the results of an experiment are presented which show that the majority of the radiation outcoupled from statistically rough devices is derived from the 'fast' surface plasmon localised at the outer metal surface.