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. The instrumentation used on present day devices is shortly reviewed, with special attention to the active spectroscopic techniques of laser induced fluorescence and Thomson scattering.