Sonochemistry is the use of ultrasound to enhance or alter chemical reactions. Sonochemistry in the true sense of the term occurs when ultrasound induces "true" chemical effects on the reaction system, such as forming free radicals which accelerate the reaction. However, ultrasound may have other mechanical effects on the reaction, such as increasing the surface area between the reactants, accelerating dissolution, and/or renewing the surface of a solid reactant or catalyst. This comprehensive review summarizes several topics of study in the sonochemical literature, including bubble dynamics, factors affecting cavitation, the effects of ultrasound on a variety of chemical systems, modeling of kinetic and mass-transfer effects, the methods used to produce ultrasound, proposed cavitation reactors, and the problems of scaleup. The objective of this paper is to present a critical review of information available in the literature so as to facilitate and inspire future research in the field of sonochemistry.