Hydrodynamic cavitation reactors are known to intensify diverse physical and chemical processes. In this article, flow regime maps have been proposed that give an overview of the operation of hydrodynamic cavitation reactor for different combinations of design and process parameters. These maps are based on simulations of cavitating flow using mathematical model that couples continuum mixture model with diffusion limited model. Specific flow regimes have been identified depending on the energetics of the collapse of cavitation bubble as sonophysical, sonochemical, and stable oscillatory (no physical or chemical effect). The radial motion of the bubble in the cavitating flow is governed by the mean and turbulent pressure gradients, which in turn, are decided by the design parameters. An analysis of variations in the pressure gradients in the cavitating flow with design parameters has been given. The flow regime maps form a useful tool for identification of most optimum set of design parameters for hydrodynamic cavitation reactor for a physical or chemical process. (C) 2012 American Institute of Chemical Engineers AIChE J, 2012