The volume opens a series dealing with the optical spectra of rare earth ions in inorganic solid rare earth compounds, host crystals, and glasses. Ions of the elements Ce to Lu in host materials serve as optically active centers in commercially used phosphors and in solid state lasers. Thus, much experimental and theoretical work on their spectra has been performed and is still in progress. Both a phenomenological description of the experimental spectra as well as the appropriate energy level diagrams are given. The topics of the present volume are the optical spectra of cerium and praseodymium ions in solid host materials. Especially the spectra of Pr3+ ions continue to be studied intensively because of laser activity in the visible region and the experimentally observed upconversion phenomena. The great variety of host materials includes praseodymium halids, PR3+-doped rare earth halides like LaF3 or LaCL3, fluorite, garnets, borates, phosphates, double nitrates or glasses.