We provide a brief, introductory overview of current efforts to
develop efficient multiphoton absorption materials. The central
role of organic and metal-organic dyes characterized by excited
states with intramolecular charge transfer character is emphasized and the basic features of essential state models frequently employed to rationalize one-photon and two-photon absorption (TPA) properties are outlined and critically discussed. The use of different strategies for enhancing the TPA cross section is exemplified by resorting to some recent results of collaborative research work carried out in our laboratory
