A spectral line emitted by an atom or molecule (throughout the article, spectral lines seen in emission ere described; however, the theory applies equally well to the absorption of radiation in plasmas) has a finite width and there are three distinct physical processes which contribute to this broadening. The energy levels between which the transition takes place have a natural width, which arises from the reaction of the radiation field on matter. This means that an atom has a finite time in any level before sponteneou decay takes place. Also the emitting atom may be moving in the line of sight, which produces an apparent change in the line frequency through the Doppler effect. Finally, broadening of the spectral line is caused by the interaction of the emitting atom with surrounding particles in the plasma. Thus a detailed analysis of line shapes can yield much information on the density, temperature and motion of plasmas, whether they are high density plasma created in the laboratory, or the lower density plasma of interest in astrophysics, such as the solar corona or the interstellar medium.