Visible and ultra-violet emission spectroscopy is a well established plasma diagnostic technique extensively used in contemporary fusion experiments. Theoretical plasma models are required to derive the relevant physical parameters. These models are reviewed in the first part of this paper. They allow spectral line intensities and radiative power losses to be calculated from the knowledge of the detailed atomic physics processes occurring in the plasma. In tokamak plasma experiments, impurity contamination and transport are important concerns. Basic spectroscopic methods used in their understanding are described. They include the determination of impurity concentrations either by line emission modelling (through the use of an impurity transport code), or by direct charge-exchange recombination measurement. They also include the evaluation of neutral particle fluxes at the plasma periphery. Finally, the experimental techniques used in the derivation of impurity transport coefficients are reported.