Publisher Summary This chapter discusses the practical significance of optically induced damage and multiphoton absorption. Optical damage is important because it limits the maximum amount of power that can be transmitted through a particular optical material. This damage imposes a constraint on the efficiency of many nonlinear optical processes by limiting the maximum field strength that can be used to excite the nonlinear response without occurrence of optical damage. This chapter presents a detailed description of several mechanisms related to optical damage including linear absorption, Avalanche breakdown, multiphoton ionization or multiphoton dissociation of the optical material, and direct (single cycle) field ionization. The chapter summarizes some of the basic empirical observations regarding optical damage. The multiphoton absorption process which constitutes a nonlinear loss mechanism that can limit the efficiency of nonlinear optical devices such as optical switches is also discussed. A calculation of the rate at which multiphoton absorption processes occur is also carried out. The theory of single- and multiphoton absorption and Fermi's golden rule are also presented in the chapter.