Several formalisms for acoustic scattering from periodic surface roughness are reviewed, compared, and a variety of numerical results are shown. For each formalism, the scattered pressure field is expressed as a sum over a spectrum of Floquet plane wave modes, a T-matrix is calculated, and used to relate the spectral amplitudes of the scattered field to those of the incident field. An integral equation formalism is developed and used to calculate scattering from an arbitrary two dimensional nondifferentiable periodic roughness profile function for both Dirichlet and Neumann boundary conditions. The roughness profile function is approximated by truncating its Fourier series representation. This approximation provides a continuous and differentiable representation of the roughness profile function and guarantees a convergent solution for the surface field. Then a formalism based on Green’s identity is developed and used to determine the effect of the approximation on the scattered field spectral amplitudes. Numerical results are shown and demonstrate that the spectral amplitudes of the plane wave modes scattered from a nondifferentiable roughness profile function and from its truncated Fourier series representation are essentially identical over a wide range of incident grazing angles, wave numbers, roughness amplitudes, and roughness types, and that the integral equation formalism may be used to calculate scattering from very rough nondifferentiable periodic profile functions. Then scattering from a three dimensional fluid-elastic interface is considered and an integral equation formalism and a formalism based on the null field hypothesis are developed. Exact closed form solutions for plane wave scattering from sinusoidal and triangular surface roughness are obtained. The T-matrix that follows from the null field formalism is shown to be considerably simpler than the T-matrix that follows from the integral equation formalism and it is used to obtain a variety of numerical results that show some of the  effects of mode conversion on scattering from a rough fluid-elastic interface. Then it is used to calculate scattering a linear frequency modulated pulse from three dimensional sinusoidal and triangular surface roughness.