A brief introduction into the spectroscopy of fusion plasmas is presented. Basic principles of the emission of ionic, atomic and molecular radiation is explained and a survey of the effects, which lead to the population of the respective excited levels, is given. Line radiation, continuum radiation, opacity and line broadening mechanisms are addressed. To access the core of a fusion reactor, active spectroscopic techniques have been developed, of which charge exchange recombination spectroscopy and Thomson scattering are treated in some detail.