This paper discusses diagnostic information derived from the visible radiation spectrum of JET. Most of the optical line features originate from the relatively cool boundary layer of the tokamak plasma. Their importance stems from the crucial role which the edge plasma appears to play in the overall performance of the tokamak. Line radiation from the edge plasma is interpreted in terms of influxes of particles from the boundary materials. Using multi-chord viewing of the plasma cross-section, recycling rates, erosion and re-deposition of the elements on different surfaces can be followed. The separate elemental influxes and the composition of the core plasma are related through the release processes from the walls and through cross-field particle transport. Together with the chemical abundances of the ions in the core, the influxes give the global confinement of the elements in the plasma. Collision processes from the core plasma are also represented in the optical spectrum. These (qenerally weaker) features include the bremsstrahlung continuum from which the effective ion charge is routinely derived. Forbidden lines due to transitions between levels within the same ground configuration in highly ionised atoms are sometimes observed. Transitions between high quantum (Rydberg) states of plasma ions can be excited in the visible by charge transfer collisions with energetic beams of injected neutral atoms. The spectral signature of these charge exchange recombination lines in H like bare nuclei in the plasma core to be measured. Diagnostic applications of each of the above features of the optical radiation from JET are described.