As shoaling waves approach the shore, wave celerity (speed) slows, the wavelength decreases, and wave height increases. Once waves begin shoaling the infinitesimal height assumption of linear theory is often broken. In addition, the wave is steepened in such a way that linear wave theory can not accurately describe the wave. Guza and Thorton (1980) demonstrated that although these violations to linear wave theory occur, linear theory still does an adequate job ( ± 20%) of describing many of the behaviors of breaking waves. Linear theory implies that all formulas governing wave behavior are first order (no variables are raised to a power higher than 1). Non-linear theory implies that equations that are to an order greater than one are applied to wave description. By way of analogy, first order equations always describe straight lines, whereas, higher order equations describe lines with increased curvature for every increase in power. Efforts to more accurately describe waves have applied non-linear theory to the description of waves. Using complex Boussinesq equations, non-linear wave theory relaxed the infinitesimal wave height assumption and allowed for wave harmonics to interact. These new models more accurately describe shoaling waves and contain information about wave motion that is necessary to explain wave driven sediment movement. However, these models still do not accurately predict wave motions once breaking occurs (Freilich and Guza, 1984). REF/DIF 1 applies weakly non-linear theory to simulate propagating waves. Weakly non-linear theory as applied by REF/DIF 1 is based on a Stokes expansion of the water wave problem and includes the third order correction to the wave phase speed. The wave height is known to the second order (Kirby and Dalrymple, 1994). This means that REF/DIF 1 uses second and third order equations to describe wave behavior more accurately than linear theory. A more detailed explanation of the strengths, weaknesses and operation of REF/DIF 1 is described in Appendix A.