Publisher Summary This chapter focuses on the progress in the field of interaction between optical centers and their surroundings (the ligands) over the past decade. The chapter presents the models necessary for the discussion and the phenomena to be expected. These models are mainly the configurational coordinate model and the Forster–Dexter model of energy transfer. The phenomena involved are radiative and nonradiative transitions, spectral band shapes, including zero-phonon lines, as well as energy transfer and energy migration. The various developments are illustrated by several examples. The existence of optical centers in solids, in liquids, and in molecules is well known and their properties have been studied intensively. Two of these are well established, which are; (1) the phenomenon of optical absorption, leading to colored compositions and (2) the phenomenon of emission of radiation (luminescence). The chapter presents the configurational coordinate diagram that describes the interaction of an optical center with its surroundings in the absence of center-center interaction. The chapter also discusses the interaction between two centers that results in energy transfer. If luminescent centers come closer together, they may show interaction with each other that result in new phenomena.