Most of the theoretical detail contained in this monograph is confined to this chapter. Here is discussed the interaction of electromagnetic radiation, from the ultraviolet through the visible to the near infrared, with matter. The treatment of laser excitation of gases and solids through vibrational and electronic transitions, and the subsequent energy transfer or bonding modifications, is from an elementary level. Particular attention is given to the special properties of surfaces; the mechanisms of adsorption, desorption, and nucleation, so important in this area, are discussed. A more practical review of some useful modelling work in the areas of laser induced heating and the various reaction modes and film growth rates induced by lasers is also presented.