A critical review is presented on the state of knowledge and calculation capability for coastal overwash. Overwash and overwash deposits (washover) accompanying hurricanes and severe storms can devastate coastal communities and habitat, but in many areas these processes are essential for maintaining the integrity of barrier islands while creating new habitat. This review covers general studies of overwash processes, studies from a geological perspective, physical modeling, field studies including measurements of washovers and related hydraulics, and the state of numerical modeling capability to predict overwash. Although significant literature exists describing individual overwash events and locations experiencing frequent overwash, complete hydrodynamic and morphologic documentation of an overwash event is lacking. A limited number of algorithms or models exist to quantify overwash occurrence, deposited sand volume, and upper beach profile evolution. Existing models of overwash occurrence and one-dimensional beach profile evolution have been shown to perform successfully against available data, and areas of improvement are identified. Models must be made capable of simulating the various washover morphologies that have been produced by different hydrodynamics, overwash spreading based on dune topography, friction and percolation, and interaction between swash bores. Comprehensive laboratory and field data sets to achieve these aims are still lacking.