The focus of studies of OTEC physical environmental impacts is the determination of OTEC-induced modifications, at several scales, to the ocean environment. Predictions of physical modifications and of the transport and mixing of plant effluents are required for broader ecological assessments. Modifications to ocean temperature structure also impact power plant operations, and maintenance of the thermal resource must be accomplished in a manner consistent with environmental considerations. Physical model studies at laboratory scales have provided data on the potential for resource degradation as a function of plant design and receiving water characteristics as well as the pathways and mixing of plant trajectories. Continuing physical model studies explore near-plant effluent behavior further for variations in upper ocean stratification and currents. Predictions of intermediate-field spreading and mixing are necessary to estimate impacts in the far wake of a plant and bear on spacing between plants. Analytical and numerical models for this regime are presently under development. The results of near-plant and intermediate-field effluent behavior provide the input and boundary conditions for models that simulate the large-scale transport of OTEC perturbations. Limited-area and whole-basin numerical models in development will be employed to address such larger-scale modifications. Climatic impacts of OTEC are coupled to these larger-scale ocean modifications, and assessments of climatic impacts are dependent on the results from ocean models. Predictive models, both physical and mathematical, are only as good as the physics and data underpinning them. Physical oceanographic data in potential OTEC site areas are necessary both for characterization of the input parameters for many of the modeling efforts and for evaluation of the model simulations.